Fat Girl

Antipholus of Syracuse: Then she bears some breadth?

Dromio of Syracuse: No longer from head to foot than from hip to hip: she is spherical, like a globe; I could find out countries in her.

—The Comedy of Errors, III.ii

A character who's at least overweight and Always Female who is portrayed as being either insecure, unimportant or both. Often, she functions as a source of comic relief, whether or not the subject is her weight. Like the Black Best Friend and Pet Homosexual she is the unconventional one who is, likely the all-American main character's best friend for her size. Either that or she's the main character of a drama where her big conflict is having an issue with her weight. She can range anywhere from Hollywood Pudgy, where the actress is actually fairly thin and her fatness comes across as more of an Informed Flaw than anything else to BBW, where large women are portrayed more positively. When the main character is a Big Beautiful Woman that is an inversion and should simply be listed there. A common characteristic of Brawn Hilda and the Grande Dame.

Examples of Fat Girl include:

Anime and Manga

Sailor Moon had an overweight friend who got Brother Chucked pretty quickly. Her name was never actually given, not even in the extra material (even though most of the extra characters are), so most fans simply call her what the credits did -- "Usagi's fat friend".
- Sailor Moon Abridged has a field day with this poor girl.
The main character of Real Drive has two friends, a lithe and athletic one and a fairly pudgy one. However, the female body shapes in the series are actually surprisingly realistic, so the protagonist weighs enough that she could probably pass off as a Hollywood Pudgy Fat Girl in another series herself.
Kanae in Ojamajo Doremi. She's rather well adjusted, though does try to lose some weight in one episode.
Tamako in Silver Spoon. The manga wastes no time in pointing out the Punny Name fitting her egg-shaped body.
Nago-san in Daily Lives of High School Boys is probably just chubby, but a convex mirror shows that she can look much nicer if she loses some weight and not frown as much.
- And later on we see another girl who bears a homophonous name, go to the same school, and looks exactly like a cleaned up Nago. The only disambiguation is their kanji.

Comic Books

Wonder Woman's sidekick, Etta Candy, in the '40s. Woo woo! Etta is a notable aversion for being something of an Action Girl, not the slightest bit insecure, and frequently important to the plot; although still serving as Plucky Comic Relief. Since the dawn of the Silver Age, she's been relegated to the Scrappy Heap, and her rare reappearances usually either depict her as insecure (since obviously a plus-sized woman cannot possibly be confident or happy), or else emphasize her Action Girl status while slimming her down a fair bit.
And Richie Rich's pal Little Lotta in the 50s. Harvey Comics never met an Anvilicious name they didn't like. She is portrayed very positively, much of her material built around her near super-strength.
Millie the Model's friend Daisy is a relatively happy and well-adjusted example of the type.
Suicide Squad: Amanda Waller is another example of a large woman who is not insecure about her weight, and is plenty Badass. At least, she was, until the New 52 reboot and the Green Lantern movie got a hold of her and retconned her into having a more 'normal' appearance.

Film

Anaïs from A Ma Soeur!, which is known, in English, sure enough, as Fat Girl.
Rosemary Shanahan from Shallow Hal, played by Gwyneth Paltrow in a fatsuit.
The title character of Precious. She, however, absolutely doesn't function as comic relief.
The big girl in Take the Lead is creatively named... Big Girl.
Gluttony from Bedazzled.
As Rufus T. Firefly says of Grande Dame Mrs. Teasdale in Duck Soup:

I can see you now in the kitchen, bending over a hot stove. But I can't see the stove!

Martha `Dumptruck' Dunstock from Heathers.
Michelle, one of Andy's best friend's in How to Lose a Guy in 10 Days, who believes she's fat inspires Andy to write an article that is a How to in reverse about dating.

Literature

Agnes in Discworld has aspects of this, but only from an outside perspective. In fact, given that a major aspect of her personality is how irritating she personally finds the trope, it probably counts as a Deconstruction.
Brittney from Gone, books 2 and 3, although she is not comic relief and her weight is only mentioned once.
La Falvine from Malevil is an old fat woman. She is given considerable grief from a rival old woman who insists she is a burden to their limited food supplies after the Apocalypse.
Carmen of The Sisterhood of the Traveling Pants is a subversion. Her size and shape are an issue mostly because her Latina curves make her stand out from her father and his new family as seen when she gets so upset when her bridesmaid's dress doesn't fit because it was clearly designed for someone skinny. There's also the fact that she's the narrator and the one who holds the group together.
- There's a racial element to it, too - one of the reasons why she gets so upset when her bridesmaid's dress doesn't fit is because it was clearly designed for someone like the newer, blonder family that her (also white) father seems to be replacing her with - a family that never got Carmen's Latina bottom genes.
Lois Waller of the Sweet Valley Twins series is a shy, overweight student who was once friends with popular student Janet Howell but now the only person who's nice to her is the kind-hearted Elizabeth.

Live-Action TV

Zev from Lexx grew up this way. She didn't have a lot of room for exercise at the Divine Order's factory farm for brides.
Lyndsay in Teachers. Notably she does get a romance storyline with Matt, the most conventionally attractive member of the cast, but only after an episode of him being shocked by his own attraction to her.
The ABC Family show Huge contains many fat girls (in fact most of the cast is overweight), but completely averts the stereotypes of the trope.
Lauren Zizes on Glee. She is presented as a really "cool" character but still often functions as the comic relief, mostly referencing eating.
- Lauren is more of a subverted Brawn Hilda than this trope.
Kim from Kath and Kim. Despite thinking she is the epitome of sexiness and slimness, and often squeezing herself into far-too-small clothing, the other characters have no qualms about pointing out how chubby she is. Especially her own mother!
- Also Sharon, who was played by the (at the time) obese Magda Szubanski. It is speculated that Kim keeps her around to make herself look better by comparison.
Laura Quinones from Carrusel
Harper from Wizards of Waverly Place is the best friend of the show's main character Alex Russo and is often known for her outrageous fashion ensembles that she designs herself. She is very optimistic, though a little insecure at times but not for her clothes which are generally non-figure-flattering.
Sookie St. James from Gilmore Girls is the loveable control freak and scatterbrain/klutzy best friend of Lorelai Gilmore.
- Something of a subversion; while Sookie is skittish, her weight is never mentioned by anyone, she's never seen as fretting about being on a diet, and despite being a chef there's never a single "Ho ho, she should stop sampling her own products" crack made. The character could have been played by a woman of any size without having to be re-written (not that anyone but the awesome Melissa McCarthy should have played it), making her far from the usual "fat girl" type.
  - And yet sadly, when is the awesome Melissa McCarthy ever seen playing the main character where her size isn't an issue?
Theresa 'Terri' McGreggor of Degrassi is an insecure, overweight girl. Both Ashley and Paige's strong personalities completely dominated hers to the point to where she really could not stand up to them.
Darlene from 1000 Ways to Die‍'‍s segment "Die it". She's so desperate to stop being this that she orders tapeworm larvae and swallows them so they'll eat up anything she eats. But since the tapeworm breed and spread inside of her, two months later Darlene loses both 27 kgs... and her own life.

Music

The song "Huggin' And Chalkin'" (recorded by numerous artists but most famously by Hoagy Carmichael) is about a man in love with a woman so fat he can't hug her in one go -- he has to hug her in increments.

Theater

Mabel is a Fat Girl in The Pajama Game. That's exactly what the script says.
Nell the kitchen wench from The Comedy of Errors, as the page quote suggests.

Video Games

Eunice, the innkeeper's daughter in Rune Factory Frontier. Notably, you can marry her.
Hilda in Shadow Hearts: From the New World in her curvy form. She has a hyperactive metabolism, and changes between curvy, slim, and bat forms depending on what she eats.
Princess Spumoni from Little King's Story. Almost everything she says revolves around eating. She even has a side-quest that involves gathering ingredients.
Princess Theredras, in the World of Warcraft dungeon Maraudon. She's also butt-ugly. Her mother, Therazane (ruler of all earth elementals) is almost completely spherical.
- Many players make jokes about dwarf and tauren women being overweight. Oddly, the player consensus is that the human female model is also overweight, even though she's really just an example of Hollywood Pudgy.
  - This has led to something of a community backlash from those who don't hold that anyone who isn't a night elf or blood elf is overweight in recent years. Ditto for female dwarves, though not quite to the extent that female humans get defended.
The protagonist of Otometeki Koi Kakumei Love Revo is an overweight girl (as in 220 lbs) who used to be pretty enough to win beauty pageants, until her love of sweets did her in. The point of the game is to lose the weight while pursuing one of five hot guys (plus the pudgy childhood friend who gets hot later, and your brother).
Sort of the point of Fat Princess. Make her fatter to keep the other guy from making off with her.
A morbidly obese version of Eve appears in Dragons Lair II: Time Warp who chases Dirk mistaking him for Adam. Her appetite ends up dooming Eden.
What Ai Ebihara used to be in Persona 4. And what Hanako Ootani still is.

Web Comics

Carrie from Everyday Heroes, although this is somewhat of an Informed Attribute. She isn't drawn that much heavier than the other characters, but a few have commented on her weight. If the comment comes from a rude guy, it's Hammer Time!
Faye of Questionable Content is considered this. She wasn't drawn any different from the rest of the rather thin cast, though she and other cast members would comment on her weight, until recently; as the artist's ability improved, Faye's curves became more apparent, though arguably not unattractive. The same could be said of Marigold.
- Mostly subverted in that both are considered attractive by the other characters, largely because of their...ahem, assets. They even go out of their way to assure Marigold of this.
- However, both have other characters at some point pestering them to exercise and "improve" themselves, even though Word of God is that Faye is more Hollywood Pudgy than fat.
The Empress of Blood in The Order of the Stick. She got wind that more powerful dragons have a distinct tendency to be bigger, so she decided that the quickest way to become more powerful was to gorge herself constantly.
Marilyn "Jackie" Monroe from Meaty Yogurt, a young woman with a bad case of Small Town Boredom.
Hanae from Red String, who is portrayed in a more positive light compared to most examples of this trope.

Web Original

The Nostalgia Chick has her own BFF (Big Fat Friend) Nella, which she pays handsomely to be her fat friend so that she would look nice in comparison.

Western Animation

Mavis Cruet the fairy in Willow the Wisp. Most of the jokes are about the fact she can't fly.
Toot from Drawn Together. In one episode they even lampshaded that she just existed to be a one-dimensional fat joke.
Sadie from Total Drama Island.
Penny from The Mighty B! is the title character's clumsy, dim-witted, obese best friend, who loves taffy. Although she and Bessie are best friends, Penny also shows loyalty to Portia who is often opposing Bessie or trying to humiliate her.
In the TV series of The Little Mermaid, one of Ariel's sisters was retconned into a fat girl and she can be seen eating every time she appears.