If making fun of your ass is your boyfriend's idea of joking, his sense of humor leaves something to be desired. What's funny about putting down someone you claim to love?
Your guy has a lot to learn about women. Mature men know that making critical remarks about a woman's body is not a loving or wise thing to do. Even dumber is to do so in the form of a threat. Strike three is an attempt to defend your rude remarks with the lamest excuse ever invented for bad behavior, the ever-popular "I was joking."
I hate to say it, but I don't think your guy is joking. If he were, he wouldn't keep making passive-aggressive comments about the size of your rear end. If he only said it once, I might buy the possibility that he was kidding, but to "joke" repeatedly supports the notion that many a truth lies in jest.
So, yes, I think you have every right to be offended. I think you need to make it clear to your boyfriend--emphasis on boy--that making wisecracks about your body is unacceptable. If he really intends to dump you over it, tell him to go ahead and dump you now, because you have no interest in dating someone so shallow and passive-aggressive.
Good luck. Thanks for the question.