There are degrees of fat. I don't think anyone is asking to be so big they can barely walk. A huge dude with a belly and arms as thick as trees can be just as formidable as another guy who is totally shredded with a perfect six-pack.
This is especially true when you add magic to the equation. If a scrawny twig of a man can be a wizard or sorcerer I don't see why they couldn't survive while also being on the chubby side.
