They could just be making fun of the situation, nothing more. They could also do that just to see how you would react or how you would take up the situation. It's probably friends' way of teasing each other.
Well, that sounds pretty funny. In school, Me and some other people from my class were in a different classroom, and they had to trace each other's heads, i couldn't stop laughing because it sounds weird. I don't think they meant anything about you being fat if thats what you're thinking