You have a lot of good comeback suggestions. I would suggest ignoring the comment. An ignorant comment doesn't always deserve a response. If you say nothing, the person who made the impolite comment will be left talking to himself. The words will echo in his head and he will understand that his words were not appropriate.

My fat body doesn't hurt anyone unlike your ugly mouth. Why don't you go back to your kindergarten and relearn basic manners?

If he/she said in an unfriendly voice, you don't have to be nice, counter attack - You are sick ! I sleep well, I eat well, what's so funny skinny guy?

"It's absolutely not fat, I'm just healthy!" :)

Really, I look so fat?