Actually, this isn’t totally wrong, even if the comic is obviously a comic.

Fat people (at least, the ones who aren’t bedridden) DO tend to be pretty significantly stronger than skinny people that aren’t doing enough exercise, and it’s literally for this reason. Carrying all that fat around genuinely does provide some training.

Don’t get me wrong though, someone who’s doing proper exercise will obliterate both of them, but it’s still an interesting fact.