Miracle-Gro Monster

Lucky you, you just bought a new pet at The Little Shop That Wasn't There Yesterday. But you'd better be sure that you read the care-of manual very carefully. Odds are there is one thing that your pet should never under any circumstances get its hands on, otherwise it'll start growing...and growing...and growing, until it's a 50 Foot Whatever. Before you know it, the military will be shooting at your pet, unaware that all it needs is some Applied Phlebotinum to get back to normal.

This may be a natural part of the creature's biology, or it could be the creature became exposed to Green Rocks.

When a villain does this deliberately, it's Make My Monster Grow.

Anime and Manga

 * Moguchan, from an episode of ''Mirumu de Pon!", a plushie brought to life by Rirumu's magic, grew whenever it ate anything.
 * In the second-to-last episode of Wandaba Style, Michael Hanagata becomes enamored of Sachiko, a tentacle alien that gets bigger when exposed to water. Unfortunately, Michael takes her to play in the ocean...
 * Demashita! Powerpuff Girls Z has an episode where a duck grows bigger.
 * Love Hina has an episode where a robot turtle grows bigger the more metal it eats.
 * The last episode of FLCL has Mamimi feed a robot dog, which grows.
 * Rage Shenron from Dragon Ball GT grows bigger the more electricity he absorbs.
 * In one episode of Wedding Peach, the Love Angels fight a snow demon that grows every time Angel Salvia cuts it with her sword.
 * Happens to Nega (a tiny extra-terrestrial cat) when he is infected by Earth's common-cold virus, in the gag episode "The 'atchoo!' of horrors" of Magical Angel Creamy Mami

Film

 * Honey I Blew Up The Kid had a human baby who grew bigger whenever it was exposed to electricity.
 * The Blob gets larger the more it eats.
 * Hedorah from Godzilla VS The Smog Monster grows larger and more powerful the more it eats toxic waste.
 * Evolution: the aliens react to fire by melding and enlarging. So when the Army decides to napalm-bomb them...
 * The Amazing Colossal Man
 * The Crites from the Critters film series grow after they eat.
 * In Seven Faces of Dr. Lao, it turns out that Dr. Lao's pet goldfish has this problem if any part of it is exposed to air. When the Mooks of the Big Bad knock over the goldfish's little bowl, it grows... into the Loch Ness Monster.

Literature

 * The classic children's book A Fish Out Of Water.
 * Monster Blood from the Goosebumps subseries of the same name, along with whoever or whatever eats some.
 * In the story "The Baker's Cat" by Joan Aiken, a cat grows larger than the entire town after his owner feeds him yeast.

Live Action TV

 * Kitten Kong from The Goodies, who ate feed laced with growth hormone and started growing a lot.

Radio

 * The Chicken Heart probably fits here... it's not a WHOLE creature, but it still grows... and grows... and grows...until it is as big as Earth.

Theater

 * Audrey II from Little Shop of Horrors. The more blood it gets, the bigger it gets.

Toys

 * Gadunka from Bionicle, a small aquatic creature that grew into a large sea monster due to the Mask Of life's curse. Apparently, once it was restored to its normal size

Video Games

 * The Maw, from the X Box Live Arcade game of the same name.
 * The Munchables for the Wii. Both player characters do this, but shrink back to normal at the end of every level when they...turn everything they ate into orbs.
 * In the Wallace and Gromit game Fright of the Bumblebees, bees who who had been fed pollen from flowers that Wallace had made grow to absurd proportions by a homemade fertilizer he had created out of the active ingredients of body-builder's weight gain formula grew to enormous size, with the queen ending up the size of his truck. One has to wonder if there were any side effects from eating the honey that the bees produced from that...
 * While he's already pretty competent in the combat area, Little Birdie from Metroid: Other M later becomes
 * A Wii-exclusive powerup in Sonic Colors involves becoming one of these.
 * The titular blob in Tales From Space: About A Blob is one of these. It shrinks back down twice, but on it's third try, it grows bigger than the planet.
 * In the various Ivalice Alliance games, the Carrot is an escaped pet that when the party encounters it turns out to have mutated into a massive malboro, a recurring creature in the Final Fantasy games known for its ability to inflict multiple status ailments.

Web Comics

 * Saturno gets larger the more demon s/he eats.
 * Mentioned in a Narbonic strip, with a string of flashbacks of Artie coming to Helen with bad news, one of which is "A quantum singularity is swallowing the sealed-off section, and grenades just make it bigger!"

Web Original

 * Tasty Planet and all the 50 other similar games.
 * Leak has the main protagonist grow the more rock it eats.
 * Slime Quest has the titular slime grow bigger the more things it eats, but it can convert it's mass into upgrades.
 * Rampage, from the Global Guardians PBEM Universe, grows larger the angrier he gets... and he starts out pretty pissed off.

Western Animation

 * The Wow! Wow! Wubbzy! episode "Attack of the 50-Foot Fleegle": Wubbzy's new pet is never supposed to be fed candy, specifically because of this trope.
 * Beebo from The Powerpuff Girls was designed to be fed only once in his lifetime, but the girls ended up feeding it once each. It ends up eating half the town.
 * A hamster during an Invader Zim episode gets larger and larger the more it eats.
 * Viva Pinata episode "Confetti-itis" has this happen to most characters in the show.
 * Tank on Lilo and Stitch: The Series has this happen to him the more he eats.
 * The Courage the Cowardly Dog episode "Muriel Blows Up" has this happen to each of the main characters.
 * Happens to a little alien felon whom Guy Gardner douses with coffee in the Cold Open of one episode of Batman the Brave And The Bold.
 * Eileen aka "Birthday Girl" on Word Girl gets bigger (and greener) the greedier she gets.
 * The sponge monster from the My Life as a Teenage Robot episode "Speak No Evil" grew every time it was exposed to water.
 * Johnny Test also had a sponge monster that grew when it was exposed to water.
 * In Veggie Tales, a monster representing a lie Junior Asparagus told keeps getting bigger as Junior has to tell bigger and bigger lies in order to cover up the first lie he made.
 * There was a similar episode in which a "Rumor Weed" grew and eventually took over the town as a rumor spread and became more and more exaggerated.
 * In one Sushi Pack episode, a scientist creates a tiny dinosaur that eats trash. It accidentally gets splashed with chemicals, which makes it grow bit by bit with all the trash it eats. It also somehow gains the ability to turn anything into trash for it to eat.
 * Jimmy Two-Shoes: Yet another fine product from Misery Inc. Useful for a Clingy Jealous Girl needing to break up an alleged date her crush is on.
 * In the The Grim Adventures of Billy and Mandy episode named "Brains!", this happens to a Meteor that falls out of a sky that eats brains.
 * An episode of Chip 'n Dale Rescue Rangers has Dale adopting an alien lizard that appears to play the trope straight at first, before revealing that he is just as intelligent as the main characters. However, he loses intelligence at the same time as he gains size, and it's stated that the ancient dinosaurs were the result of a previous away team succumbing to this fate.
 * Jackie Chan Adventures had Hak Fu summoning miniature shadowkhan. They failed to prove a threat until he discovered their shadow eating abilities. It rendered victims comatose and turned them into massive landsharks.
 * In the My Little Pony Friendship Is Magic episode "Secret of My Excess", Spike becomes bigger and bigger the greedier he gets.
 * The Mongolian Fire Fist Demon from the Kung Fu Panda Legends of Awesomeness episode "Po Fans Out", which gets bigger whenever it gets hit.
 * A once-off Mad Scientist villain in Atom Ant creates a "Gobbling Glob", a small gelatinous monster (looking something like a Pac-Man ghost) which is only the size of a baseball... but grows bigger and bigger as it eats. And it's ravenously hungry...
 * My Life as a Teenage Robot