In the Jackie Chan Adventures fic Queen Of All Oni, after Jade becomes a Shadowkhan girl once more, her appetite and metabolism greatly increased, to the point that, in one chapter, she is described as eating a rib, BONE AND ALL, and has eaten large amounts in several other chapters (to be fair, she did have a big appetite BEFORE the transformation, as Season 2 episode 9 shows).
John in With Strings Attached. Eats at least three times what a normal person eats. He can ingest and draw nutrients from just about any organic substance. As a side effect, he's immune to poison and drugs, including alcohol.
Agumon was always a big eater, but in Transcendence: Digital Curse he consumes enormous amounts at some times. At one point he manages to eat a dozen or so containers of food, and he keeps eating an unspecified amount more. Justified in that case.
Haruhi, Kuyou, and Tsuruya in You Got HaruhiRolled!. The latter is implied to be able to devour at least ten times her own body weight.
Kit from the Hunger Games fanfic Some Semblance of Meaning is a Mouthy Kid in more ways than one. When asked what his favorite food is, he responds, "Everything!" And the narrative once described him as "eagerly attempting to gobble down half of the Capitol's food supply" at breakfast.
Simba eats way too much food at King Hapana's pride in Desperate Measures, developing a Balloon Belly in the process. It turns out that King Hapana was deliberately fattening him up in order to slow him down so he couldn't escape.
Haiba eats so much at Jowai Resort in Changes that he becomes morbidly obese. It also spawns his alter ego, Don Haiba.
In both cases, the obesity wears away by the next story.
The Twilight Child: Scootaloo manages to win an eating contest against already established Big Eater Pinkie Pie, as well as Rainbow Dash and Trixie.
Princess Celestia, as well. At Twilight Sparkle's birthday party a character notices her casually chowing on an extremely large wedge of cake, to the horror of the Canterlot ponies watching.
In A Very Potter Musical, Ron Weasley always has a snack, including but not limited to: a carton of Chinese food, a bag of Funions, Twizzlers handed to him by a member of the band, and a Hershey bar the size of his torso.
Ron: Accio double-stuffed!
Wheatley becomes this after GLaDOS implants his brain into a human body in the Portal 2 fic Test Of Humanity. He ends up eating everything in Chell's fridge after tasting food for the first time, and, later on, GLaDOS tricks him into eating 30 cakes. By the end of the fic, he's grown noticeably fatter.
The sequel has Chell's roomates being unable to make anything for dinner because Wheatley already ate nearly everything. And, Wheatley then admits he made a large order for Chinese takeout...most of it being for him to eat.
Mike gets away with being called fat in Pokeumans - as a Ditto he resembles a pink fleshy blob and turns into a Munchlax so he can eat more.
Choji in the Naruto fanfic Kitsune no Ken: Fist of the Fox, just like his canon counterpart. At one point, Sasuke actually takes advantage of this to convince him to fight Shikamaru during the qualifying round of his tournament—in addition to getting tickets for the upcoming Shadow Fox comic-book movie, they're told that there'll also be a buffet line, as the screening they'll get to see is that which is usually reserved for movie critics. Choji immediately jumps on board.
Yajirobe's appetite is played up in Dragon Ball Z Abridged, most notably with an FAQ video where he apparently started eating Yamcha's body and Bulma correctly fearing he might eat Trunks out of spite and/or hunger.