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In one The Beano strip of The Bash Street Kids, the already balloon-bellied Fatty is forced to play football (soccer), but as his name indicates he is not the fittest of characters. He solves this problem by playing in goal, and eating so much that he is fat enough to fill the entire goal-mouth!
In "The Day Flash Weighed 1,000 Pounds" (The Flash #115, 1960), The Flash is zapped with a super-fattening ray by the villain of the week (who then, to add insult to injury, gives him amnesia and sells him to a traveling carnival as a sideshow attraction). He's back to normal by the end of the issue, thanks to an implausibly fast (even given that this is the Fastest Man Alive) weight-loss regimen.
It involves steaming himself in a room used to dehydrate potatoes, for the curious.
Issue 62 of Gen13 features a very subtle use of this after a main character binges on junk food.
Superdickery juxtaposes two Jimmy Olsen covers (here and here) to reference this trope, though strictly speaking the two issues were published over a year apart (July 1959 and December 1960) and are not actually connected.
Expanding into a human superball was Bouncing Boy's actual superpower. Occasionally the more traditional version of the trope would happen to Matter-Eater Lad as well.
The New Universe title DP7 had a paranormal named Sponge who would absorb all the moisture in the air at a massively accelerated rate, causing her to balloon up in the space of seconds. She could purge the water equally quickly, looking like a punctured water balloon.
Secret Wars II had the Beyonder in human form pigging out on a gangster's yacht. He later vaporizes the flab in a second.
In X-Men, Storm first met Gambit while she was temporarily de-aged to about ten or twelve (a bit of amnesia, too). Early on, he took her to eat at (apparently) a friend's home; she was so hungry and the food so good that she stuffed herself, finally sitting back and commenting in a surprised way, "My belly is bigger."