People commenting on belly size

Annika
Last pregnancy I ballooned up around 20 weeks and didn't care. I had a ton of amniotic fluid (measured 23.4/24, if you understand that metric) and felt fine. I am 5'11" was about 145-150 before pregnancy and got up to about 175. After having my little one I breastfed so didn't care to lose weight. Then got pregnant (they'll be a year apart). Now I'm 20 weeks again and once more people like to let me how I look like I have twins or ask me if I'm sure I'm not further along. 
I see many women with tiny bumps and many women who protrude and carry like a blimp and I never ever think to comment on their size. Some women do the fit pregnancy and stay thin with just a toned bump and it's fine. Others retain water all over and look like a pufferfish. We all carry differently and we all have moments of self-consciousness, so I don't understand making another woman feel bad because she's carrying lower or higher or just differently. I thought pregnancy was supposed to be the one time where we don't judge a woman for her physique. 
Sorry I'm venting but I sent a picture of myself to a handful of friends today and got all sorts of comments about my size and only one "congrats". Why do women do this?