I think the vast majoirty of people that comment are just trying to find a way to say something nice about the fact that you are having a baby.

I am not going to get worked up or even frustrated by people saying I look big or small or whatever. Apart from the very few outright rude people I have heard, people mean well, sometimes we just don't think before we speak.

The belly is just there so people comment on it. It is the simplest way to bring up 'You're pregnant' again.