Right, I think you need to chill out a bit ! You obviously know your butt is NOT that big, so why do you take offence ? The guy is probably just teasing you, and he KNOWS your butt is not big. It's probably because you make an issue of it that he even noticed it ( running morning AND night seems a bit much..). My Brother used to tease me about my nose all the time because I kept saying I thought it was too big. It is not. But he just liked to tease me about it because he knew I cared. It's probably just good fun for him, don't go calling him a "fat *******" or anything insulting ! On the other hand, if he starts groping you or the comments become too much, and definitely sexist, then I guess a good old slap is "de rigueur" ! ;-) Good luck.