Bobobo-bo Bo-bobo: Suzu once ate an entire coelacanth as part of a gag; in the anime she looks heavily pregnant, while in the manga her stomach bloats to at least five feet in diameter and forces her onto her back.
Agumon gets one after eating a fish twice his size.
Agumon gets another when Tai stuffs him with food in an effort to get him to digivolve.
In the Doraemon reboot's take on the classic "Memory Bread" story, Nobi uses the Bread slices to copy the notes taken for homework. This backfires when he decides to have a snack in-between while studying with Shizuka (Sue in the dub), which makes him feel sick when he tries to eat another slice of Memory Bread. It doesn't help that Nobi's mom made a special dinner with a lot of shrimp, which Nobi eats anyway, and he ends up so stuffed that he's ready to explode.
Usually averted in Dragon Ball despite the ludicrous amount of food consumed by the Saiyans; but once, early in the series, Goku ate all the food in Master Roshi's fridge and his stomach became visibly bulged.
Happened to him again during the first tournament arc: he ate a ton of food, and his belly bulged a bit, to the point where his shirt couldn't completely cover it and his belt/sash was off.
Yajirobe suffered from a severe case of balloon belly when he ate a handful of senzu beans. They're meant to keep a person full for ten days each.
Krillin also does this with his Tenkaichi Boudokai fight with Goku by sucking in a lot of air to make himself float.
Happens with Goku while he's training on King Kai's planet. Due to him being starved during his lengthy run across Snake Way, he asks Kai if he can take a break to fill himself up and then proceeds to stuff himself. A filler episode that features the currently dead Yamcha, Tien Shinhan, and Chaozu training on his planet features a similar gag when the trio take time out for lunch.
In one chapter of Futaba-kun Change!, two characters swell up like balloons after eating a lunch made by Misaki. Her Sweet Tooth somehow crammed tens of thousands of calories into that one tiny box. Later, another character has his face expand into a beachball after one bite.
In Hell Teacher Nube, Miki once came across the wish-granting Kesaran Pasaran, and she (unknowingly) called up the "good luck" to run into a restaurant with a huge surplus. She walked away about four to five times her ordinary size, but recovered by the scene change.
In a more serious instance, Kyoko was possessed by the spirit of a hungry child, which caused a ravenous mouth to appear on her nape. Insatiable, it ate everything in sight, and when food ran out, it started going for other things until Nube exorcised it. Unfortunately for Kyoko, she gained as much mass as Miki did in the comedic example, only she had to work it off (with Hiroshi as drill instructor.)
In another chapter, Kyoko and Miki are possessed and they eat huge rice bowls getting big bellies at lunchtime.
Jewelpet Sunshine: In episode 6-A, Kanon fails to make cookies perfectly and stress-eats them all until Ruby confronts her, looking heavily pregnant in the process. In a Continuity Nod in episode 45, Kanon eats all of her failed Valentine's chocolates, which resemble her failed cookies, again looking heavily pregnant as a result.
Jewelpet Kira Deco: In episode 9, the gang finds a Rainbow Egg, which doubles in size, hatches and reveals a chick with a similar appearance to Kiichi. The chick, Kiichiro, cooks the gang food that makes them gain weight. When the chick has grown up to be a chicken, the gang is seen under a tree, having eaten food that makes them so fat they end up literally egg-shaped.
Jewelpet Happiness: In episode 46, Chiari and Ruby eat all their Valentine's chocolates, and both blow up in size as a result.
In episode 11 of Kidou Shinsengumi Moeyo Ken, it happens to everybody in Kyoto (spare at least five people) who is hit with a nikuman filled with cursed medicine that blows people up if directly contacted. Not even a bite, but any contact with the body.
Happened to Negi in chapter 56 of Mahou Sensei Negima!. Makie had prepared food for him knowing he was training for Evangeline's Apprenticeship Test. And overdid it. Then she brought out the "secret" diet technique of the gymnastics club, which went too far the other way.
Naruto has Choji, whose fighting style is somewhat based around this.
Happens to Naruto himself a couple of times, and to Naruto, Kiba, Shino, and Sai during the Shippuden episode 95 omake.
Happens often to Luffy in One Piece, being both a Big Eater and a Rubber Man allows him to literally become spherical after a particularly large meal (Curiously, it is sometimes averted, logic be damned). Also, during the timeskip training, Usopp also ate so much that he rivaled Luffy in size.
Also done by Kumadori, when Chopper temporarily takes him out of the fight by locking him in a fridge. He consumes all the food in it and comes out very fat. Then he uses a special technique to immediately absorb all his fat and get skinny again.
This happens to Totoko in Episodes 24 and 69 of the 1988 anime adaption of Osomatsu-kun; both times from pigging out.
From Panty & Stocking with Garterbelt, Stocking receives this treatment in "The Diet Syndrome". After having apparently gained weight, Stocking goes to extremes to keep her weight down, but to no avail. This is due to a possessed cake she was eating throughout the episode. She snaps when her belly gets big enough to look like she's pregnant, and in a fit of rage she stuffs her face with cakes and takes it to eleven.
Both girls in "Nothing to Room". It's the page image.
Many characters in Pokémon. In particular, May's Munchlax every time it eats a special Pokéblock.
Team Rocket had ENORMOUS bellies in the episode after the Summer School arc, which was a part of their Villainous B.S.O.D..
Jessie looked like she was pregnant after she, James and Meowth stole a bunch of food and Dawn's Cyndaquil egg in the Johto Festival episode.
So far, this has happened to Ash twice, once in Sinnoh (along with Pikachu) and once in Unova.
The same happened to Ash's Pikachu (four times) and Fletchling in Kalos.
Subverted in "An Appetite For Battle", when Clemont's Chespin gets this after gorging himself on macarons, but his pudginess stays with him even after the eating scene ends, and he spends the rest of the episode trying to work off his extra pounds.
The titular character of Potemayo during her regular eating binges.
This happens to Mugen in Samurai Champloo, on the few occasions when he gets to really stuff himself.
Fuu takes it one step further - her entire body changes shape, making her look quite obese for the next scene. In one episode, two thugs specifically looking for her didn't even recognize her because of it.
This doesn't happen to Toriko often because his body's Gourmet Cells can store millions of calories, all of which he needs to use his special attacks. He did get one after eating an entire mountain of pudding – and seconds later the belly was gone.
In Episode 13 of Urusei Yatsura, main character Ataru and secondary characters Cherry (an old monk) and Sakura (his niece, a very shapely and attractive woman) participate in a beach restaurant's eating contest. Ataru and Cherry quickly get "Balloon Belly" and give up, but Sakura literally eats the restaurant out of business, even after consuming diet pills. And she's still hungry afterwards, yet she somehow manages to stay looking completely normal. The reason why she's capable of this is never explained for comedic purposes.
The ending of that story was changed from the manga. In the manga version, the story plays out the same... but then she lies down on an inflatable pool float... which instantly sinks, dumping her in. In other words, while the food doesn't affect her body shape, she still gets heavier.
Also in Episode 23 where Ataru, Lum, Shinobu and Mendou – lost in a cave system that somehow led into a crashed alien spaceship – all dine on tins of alien food they found. It doesn't affect Lum, but the three humans all end up with Balloon Bellies, with Shinobu suggesting the food has expanded inside their stomachs and Lum noting that it mustn't be suited for human constitutions. When they realize they are literally ten feet from the rest of their class, they try to reach them, but fail because they're too engorged to fit through the exit tubes.
In the second episode of the hentai OVA Vixens, Doreimon gets one after she stuffs herself on sweet potatoes.
In Part 2 of JoJo's Bizarre Adventure Joseph Joestar tries to kill (or at the very least grievously maim) the Pillar Men with a stick of dynamite to buy time for his friends. The eldest of the trio, Esidesi, picks up the stick and swallows it whole. His belly winds up blowing up like a balloon. It goes to show Joseph (and the viewer) that the Pillar Men are a lot tougher than Joseph has ever face before and since.