I think the bottom line is, you probably look better than all of them and they are just jealous.
There are just average people and there are people that want to try to be the best they can. Just becasuse they have no desire, dedication, motivation or inpiration to try to get in shape or get big, does not mean you dont have to. That would be like you fucking with you mom about cooking (or something she really likes) all the time instead of going to school and not being so dumb. It just plain rudeness and they are doing it for the reaction.
I come from a family of 10 kids and I am the youngest, it is just their way of trying to control/placing their will on you.
I would just through a couple one liners at them, the next time they say it to you, like: why dont you get a better job, so you dont have to live in this dump, or why dont you go on a diet and get rid of that lard ass and cottage cheese thighs. Hurtfull, yes, but no different psychologically than what they are doing to you and then they should get the point after that.
good luck.