This guy sounds like a complete jerk who is not worth your time. I would probably just ignore him or tell him to shut up, but I'd like to think that I would find something rude to call him back if I were in your position. Does he have a big nose or something? No,that's not very charitable, but maybe it will drive the point home that it's not acceptable -- particularly as an adult -- to make fun of others' "flaws."

I would also have to agree with TC -- as a man who weighs less than 200 lbs, I doubt you even qualify as "fat." (Unless you're on the short side, maybe.) What an insensitive tool.