Consider the following:
Genji’s cybernetic body synthesizes nutrients highly efficiently, but requires so much sustenance to function that he has to eat a lot anyway
It’s not uncommon to see him snacking, though he never eats on the go. He wasn’t raised in a barn, after all
Prefers healthy, organic food but won’t turn down a chocolate donut…or eleven
A darling at Reinhardt’s weekly barbecues because he always comes back for thirds and sometimes fourths and isn’t afraid to try the really weird German sausages
Has accidentally eaten one of Mei’s science projects. Became the science project, had to spend hours in her lab while she studied the polar nematodes in his tummy. (Hanzo, from a safe distance: “So, I am an uncle?” Genji threw a beaker at him)
Speaking of which, Genji carbo-loads the night before missions, often by himself at late hours so he doesn’t hog the dinner table. Hanzo sometimes visits him and they have Important Brotherly Talks
tl;dr Genji can empty a fridge and not gain a single pound
I support Genji Shimunchies