Apart from this being a total pet peeve of mine, I've just realised something! I've started noticing that it's those people who've never been pregnant who tend to comment how big my bump is, how much it's grown, and the occasional "OMG YOU'RE HUGE!" (how anyone can think this is something you want to hear is beyond me..). It's those people who have been pregnant themselves who say I've got a cute bump or say I don't look very big for my dates.
I'd love to have a good comeback for the people in the former category though. I love the Mumsnet classic "did you mean to be so rude" but I kind of feel like I could only get away with saying that to people I don't know, if a friend comments I'd feel awful saying something snarky like that!