Luigi's years of hard partying finally caught up with him.
Breakers Revenge has Maherl's hilarious BALLOON-O! attack, in which he inflates and attempts to slam his opponent into the ground.
Data East's Diet Go Go has food-based enemies that throw food at our exercise outfit-clad hero and heroine. One hit with projectile food makes you fat, two makes you obese, leading to instant death. Averted if the enemies touch you directly, which is also instant death. To boot, the entire game's premise is about an evil scientist who's spreading junk food around the world, making people obese.
Dig Dug involves inflating enemies until they burst.
Cheng Sinzan from Fatal Fury already has a nearly spherical torso, but he can plus inflate himself with air during at least one of his attacks.
Fat Princess is a downloadable real time strategy/action hybrid that has this as a strategic element. Fattening the princess up will make it harder for the opposing army to move her. Seriously.
The game received criticism on its announcement, as people thought it was mocking the obese.
The Hebby Repp, Sudo Neku, and Toximander dream eaters in Kingdom Hearts 3D get a brief case of this whenever you feed them one of the various oversized portions of Rare Candy. Evidently, their Hyperactive Metabolism doesn't kick in instantly like it does for the rest of the dream eaters.
Though it's hard to tell thanks to his anatomy (or lack thereof), Kirby presents an especially absurd example. His size quite literally doubles when he inhales an enemy. (However, it swiftly returns to normal once he decides to swallow.)
King Dedede is prone to this, too. When he swallows Kirby, his belly gets so big it forces his head all the way back until he spits him out. He also displays it if you let him beat you in Gourmet Race.
All doubt is removed in the Wii installment, in which Kirby's belly wiggle is lovingly animated.
The Lost Vikings includes an air pump which balloonizes the three characters and make them float.
Nabnut the squirrel gets so bloated that he cannot move after eating too many acorns in Banjo-Kazooie.
In Mario & Luigi: Bowser's Inside Story, one of the Mario brothers' attacks, Snack Basket, involves stuffing Luigi with treats until he balloons up, followed by Mario tossing him at the ground to create a shock wave, as depicted on this page's image.
Also in Bowser's Inside Story is this little plot point. Guess all those fat jokes turned true, Bowser.
Come to think of it, Superstar Saga often did it with water or Chuckola Cola.
In the Metal Slug game series, if you collect enough dropped food items, you transform into an obese version of yourself. This affects not only your movement, but also your attacks: They're all increased in size and power, and the homing missiles home in on enemies better. It also changes your standing melee attack into a fork stab and crouching melee attack into a belt whip.
The Zamtrios uses this offensively, puffing out its belly if it manages to swallow you, afterwhich it'll start hitting it against the ground to cause damage to you and anyone else in the vicinity. Further, if you provoke it by breaking its ice armor, it'll temporarily inflate itself to point of looking more like a Blob Monster then a shark, switching to bouncing and rolling attacks instead of its usual biting and clawing as long as it remains in that state.
In Rayman Origins and Rayman Legends, both the player characters and enemies swell up like balloons when they are hit enough times, and pop like such when they die. The former has Globox have this when riding on mosquitoes.
Hilde from Shadow Hearts: From the New World has two forms. Curvy Hilde is the trope. Slim Hilde is the opposite. Oddly enough, she gains and loses weight via feeding (she drinks calories, not blood—some enemies have "negative" calories).
This happens to Sims in The Sims 2, in both directions: Immediately after getting up from a meal or getting off the treadmill, they may balloon to tubbyness or shrink to svelteness with a cheery sound effect. Contrast with The Sims 3, where fat and muscle are tracked separately, and gained or lost at a gradual rate.
In fact, this happened to Wario in most gamesthat feature him. For example, in Wario Land 3, eating a single donut would cause him to balloon to sumo proportions, which would then be useful for crushing weight-related blocks.
WarioWare DIY Showcase takes this Up to Eleven, as one of the minigames has Wario's belly inflating to deflect rockets. Yes, you read that right.
In Super Scribblenauts, fattening foods will turn anyone who eats them instantly into "Fat", "Chubby", and "Obese" versions of themselves, which involves simply stretching the body portion of the sprite to be wider. Using a potion with one of those adjectives will achieve the same effect.
During the Arizona chapter of the fourth Tengai Makyou game, a cinematic shows an obese cowboy sneaking into a woman's room and force-feeding her cookies that cause her to instantly fatten up. It Makes Sense in Context (though not much, since the Tengai Makyou series is full of very Japanese humor).
This happened in Um Jammer Lammy after the title character gets free pizza after putting out a fire at a pizza shop. She gorges herself so much that she looks pregnant, but she just as quickly digests it.
The Pakkun Lizards in the World of Mana games typically get a case of this if they succeed in catching someone with their signature move, thanks to them being diminutive in size compared to their victims. It's a case of tactical suicide, as they can't move or fight back while in this state, leaving them at your mercy (Plus the devoured character only takes damage when spat out, if at all).
Yume Penguin Monogatari ("Dream Penguin Story"), a KonamiPlatform Game where you play a penguin trying to lose weight to impress his girlfriend. Falling in water will instantly make you as fat as you can get, and all the enemies throw food at you which will quickly fatten you up. One level is set in a giant cake. A piracy check, when failed, will cause you to instantly bulk up as soon as you're hit.
In The Legend of Zelda: Spirit Tracks you get the boss Fraaz, whose attack involves inflating himself with fire or icy air, and he shows off his ability within seconds of you meeting him for the first time.
Shiftlings makes a game mechanic out of this: you play as a pair of bumbling space custodians, one of whom has swollen to enormous size after drinking a bottle of "the galaxy's fizziest soda". The pair must traverse various stages while connected to a hose, through which the player can transfer gas to change which one of them is bloated up and which is normal-sized.
In Sid And Als Incredible Toons, Chow-Man's torso turns spherical after eating food that happens to fall into his tray. He quickly deflates as he walks it off on his treadmill.
In Mortal Kombat II, Kitana's "Kiss of death" causes it's victims to painfully inflate and explode.
A similar fatality in Mortal Kombat 9 has Rain conjure a large ball of water over his victim's head and force it down their throats, causing their body to bloat up until it bursts. (Oddly, there is very little gore in the actual explosion)
It's never shown but during the Miss Tenku contest DLC in Super Robot Wars V, Salia states that Mao apparently drinks so much at night that she gets this. It's not out-of-character for Salia to exaggerate about something like this when she's talking trash in a contest, but Mao's reaction seems to indicate that it's true.
Mao: You truly are pathetic. Could it be all the stress of the captain role you don't deserve?
Salia: I may be stressed, but that's better then you, drinking away your cares every night until you've got the figure of a beer keg!