Virtually every major character in the Superman Universe experienced sudden fatness at one time or the other, but when it came to dreaming big, nothing else matches the great Smallville Expansion of Adventure Comics #298. In "The Fat Boy of Steel," the population of Superboy's home town grows fat overnight. The cause: an experimental cyclotron next to a milk processing plant that's exposed the milk to a "peculiar radiation." (Hey, if the bite of a radioactive spider can turn Peter Parker into Spiderman . . .) Clark (Superboy) Kent unknowingly imbibes the ultra-fattening beverage in front of the ever-observant Lana Lang, so he's forced to transform himself into an equally obese Smallvillian in order to protect his identity.
Returning home after a week's vacation, Clark and his adoptive parents are shocked to see the town has startlingly changed.
Attending high school in his secret identity, Superboy witnesses the effects of this transformation in action: fattened girl-next-door Lana Lang, interestingly, now displays a cavalier attitude toward dieting.
Because he's been witnessed drinking the irradiated milk, the boyish Kryptonian uses red kryptonite to turn himself into a BHB of Steel.
The Red K works its magic, and our hero is fattened while rescuing a trapped Lana in a phone booth. To hide the fact that he too has changed, (the booth's glass having conveniently turned translucent) he calls superdog Krypto to help him out at the last minute.
Lana has long suspected that Clark is Superboy, so she rushes over the next day to confront Clark, with mildly comic results.
Now that he's appeared in public as a fat Clark, our hero has to mask his own fatness - even as the day-to-day demands of superheroing remain. His solution: a series of disguises that the townspeople choose to accept not as ridicule but as expressions of super sympathy. (Note the fat townspeople in the foreground.)
This is enough to fool everyone but the ultra-suspicious Lana, of course.
Superboy appears at the Smallville Bank in a Santa Claus suit, there to promote the bank's Christmas Club (no, I don't know what this has to do with superheroing either!) Lana punctures the Santa suit with a large pin, fully expecting the pin to bend against the Boy of Steel's fat flesh. Fortunately, the effects of the Red K have already worn off, so all that's left is for our hero to do some "super-calculations for an antidote to counteract the isotopes which made the people of Smallville overweight!" This he quickly accomplishes with his usual super efficiency.