Renge Inui from Airmaster gains one multiple times in the series, being the Big Eater she is.
Used without anything edible in Bobobo-Bo Bo-Bobo, involving a giant toilet. Whoever falls in, they swell up, and at one point, Jelly Jiggler (who fell into the toilet with Gasser) blows up into pieces. He got better.
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.
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.
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.
Hanaukyō Maid Tai episode 10. Happens to Taro after he's stuffed with food during breakfast, lunch and dinner.
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.
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. 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, once in the anime, Usopp also ate so much that he rivaled Luffy in size.
Luffy uses this as one of his attacks through gum gum balloon, he sucks in so much air that he's turned, quite literally, into a balloon.
Worth mentioning is his second fight with Crocodile where he ingests an entire barrel of water and turns into "Water Luffy".
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.
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".
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 BSOD.
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.
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, Ataru, the main character, and two minor characters, Cherry (an old man) 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 afterward, 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, everything's almost 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.
There was another Urusei Yatsura story where Ataru eats alien food on a spaceship. His friends tell him it may turn him into an alien, which he scoffs at. It doesn't... but it does make him instantly get such a big belly that he can't get out.
Also in Episode 23 (possibly the same as the above) where Ataru, Lum, Shinobu and Mendou all dine on tins of alien food after getting stuck in a spaceship. 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.