I think alot of people make comments about the size of the bump simply as a topic of conversation, or because they don't have much experience of people being pregnant. I really cringe at the things I used to say to pregnant women when I was younger!! (about the bump, big bump, neat bump, girl, boy, etc), but I'd never known someone my age get pregnant, so I had no idea.
That said, all bets are off when it's comments about your actual weight, or other parts of your body. EVERYONE knows that that's just basic manners.
I recommend that you come up with some retorts for the next time it happens, and use them!
"wow, you've gained so much weight!"
"I know! I'm pregnant. What's your excuse?"
"Your arse is so huge!"
"Yes, it gets bigger to counteract the bump so I'm not constantly toppling over. What does your double chin counter balance?"
"Your face is all puffy"
"Your manners are non existent"
"You got fat"
"I'll lose the weight after the baby, but you'll always be an arsehole"
etc etc.