I want to punch my SIL in the face

So I really need to vent and I'm sorry but I have to do it where nobody knows me so I don't hurt anyone's feelings. 
My SIL is a heavier gal. She doesn't eat healthy and has said she never will. Instead she does all these stupid products by level thrive. Last night at my in laws house she said "wow for the first time I actually feel smaller than you". This hurt my feelings extremely bad. I'm 9 months pregnant, swollen pretty badly and before this I honestly felt great about the way I looked. I work out daily and eat really healthy. Even though I've gained more than I needed to in this pregnancy, I don't have any reason to be concerned about bouncing back after pregnancy. It didn't help that before she said that her daughter said "holy auntie your belly is huge" and she has said numerous other things to that matter. I guess I'm pissed because I know she weighs more than me and she is 6 inches shorter than me but yet because her thrive makes her feel good she felt it necessary to put me down, and in my most vunerable state too boot. I've been crying about it constantly. I definitely want to say something to her but at the same time I want to avoid her altogether. Ugh! Why do people have to say anything about pregnant women's bodies at all? It should be common knowledge that you tell them they look good, even if they don't, right?