(This’ll apply to not just anime and cartoons, but video games as well.)
Here’s the following scenes I’ve managed to spot:
Steven Universe: There’s two, and one’s recent. One where Amethyst eats a cloud and puffs up, and one where Pearl becomes spherical while she and her fellow Crystal Gems are travelling through space.
Lilo & Stitch: Frenchfry fattens up all the locals in Hawaii, including the title characters.
Adventures of Sonic the Hedgehog: Sonic and a rat that controlled his mechanical duplicate both blow up upon coming in contact with Poison Flowers. Dr. Robotnik also gets filled with water when his synthetic wife gets reprogrammed via water, and Tails gains weight before he and Sonic both turn into babies.
Totally Spies: A weight gain-focused episode where Clover (and the antagonist of the episode later on) both gain weight.
Pokemon: Diamond & Pearl Battle Dimensions: Team Rocket’s getting fat again! ding
Wario Land 2, 3, and 4: Wario becomes fat as a sort of pseudo-powerup.
Super Mario World: Both the game and cartoon feature the P-Balloon. The Mario Bros. and Yoshi also gain weight in one episode of the cartoon.
Fast Food Rampage: Fatten up vegans until they explode.
Fat Princess series: The title character (and the player characters in Adventures) gains weight through cakes.
The Sims 2, 3, and 4: Sims can gain weight if they eat too much.
Mario & Luigi 1, 2, and 3: Water inflation is a gameplay mechanic for Mario/Baby Mario, and Bowser and Luigi (at two separate points, the latter being a special move) gain weight temporarily as part of the storyline.
Pac-Man (1982): Sourpuss (Pac-Man’s cat) inflates in the second episode of Season 2, and Pinky puffs himself up in order to TF into different objects (like a pogo stick, an airplane, a flagpole, a horse, a giant, a microscope, and more).
Gramps: This What A Cartoon! short has the dog inflating via the title character’s breathing tank.
MLP: FIM: Applejack inflates thanks to Spike in Spike At Your Service.
Kirby Right Back At Ya!: Kirby inflates as part of his Bomb Kirby transformation. Also, both Kirby and King Dedede gain weight in Episode 61.
Conker’s Bad Fur Day: As part of a storyline, a gassy mouse gains weight from eating cheese, and the third piece makes him inflate until he explodes. Count Batula, as part of his boss battle, gains weight by drinking the blood of the villagers, until his rope snaps and he’s killed off, reversing the curse on Conker.
Family Guy: Peter puffs up after eating too much cereal in one episode, and Lois gains weight in another. Meg’s snake allergies also make her blow up. Stewie gains weight in yet another episode, and Brian has a fantasy where he’s fat.
The Simpsons: Homer gains weight in one episode, Mrs. Krabapple gains weight in another, and Homer has a fantasy of himself, Patty, and Selma gaining weight in yet another.
Space Adventure Cobra: A villainous genie makes a woman inflate until she explodes.
Dragon Ball Z: Imperfect Cell inflates until he explodes from a “nut shot”, one of the opponents of a tournament inflates himself to massive size, Goku kills a light-eating monster by feeding him too much light attacks, and Super Buu kills off an already-fat man by turning himself into liquid and filling the man up until he bursts.
American Dad: Hayley gains weight in one episode.
The Cleveland Show: Roberta gains weight while taking care of Rallo in one episode.
Sabagebu!: One of the protagonists gains weight.
Enma-Kun: An episode centering around inflation.
Galaxy Express 999: One episode features fat people exploding out of their own homes.
Pokemon (original series): Meowth inflates thanks to Misty’s Staryu.
Pac-Man and the Ghostly Adventures: Pac-Man inflates via a specific Power Berry.
No-Rin: There’s brief weight gain in one episode.
Panty & Stocking: This XXX-rated anime contains weight gain for one of the title characters in the fourth episode.
The Simpsons Game: Homer becomes a fat wrecking ball as a form of attack.
The Super Mario Bros. Super Show: In one episode, King Koopa blows up.
Courage the Cowardly Dog: The farmer gains weight in one episode after eating all the shredded veggies, while he, his wife, and even the title character gain weight from eating flan.
Life is Strange: The main protagonist can optionally gain weight.
Staines Down Drains: One character blows up due to her honey allergy, while some of the monsters inflate in another episode.
Zig & Sharko: Zig blows up in a few episodes.
The Powerpuff Girls (1996): The imaginary friend that the PPGs create blows up Patches like a balloon.
Foster’s Home for Imaginary Friends: Frankie’s cookie addiction really shows in an episode.
Ahh! Real Monsters: One of the main characters gains weight thanks to a parasite that causes her to eat a lot.
Futurama: Amy used to be fat as a kid, and Leela gains weight after swapping bodies with some of her co-workers. A girl also becomes fat just by pulling on her belly button ring (but only when she’s skinny).
Drawn Together: Toot gains weight in two episodes.
Pokemon: The Series XY: Ash’s Chespin gains weight in one episode.
Those are the scenes I can think of at the moment.