Fiancé's family taking it too far

Ashley • I am happily engaged to a wonderful man whom I love deeply we have been together for almost 4 yrs
Today has been a very emotional day for me I was joking to my fiancé that I had a big butt cause he asked me to scoot over so he could lay by me on the couch and cuddle and I told him I was all the way over an couldn't help it I had a fat butt. His grandfather said to me well that's not the only thing that is fat on you....I lost it I curled up on my couch and just fiancé was pissed and still is and even told him off for it. But the thing is he won't do it to his mother when she does it....she walks all over us and if I go to stand up for myself he gets upset with me....what am I suppose to do??