I think most people are just trying to be kind. And feel the need to let you know they've noticed that you're pregnant (why is it anyone else's business, especially people you don't know?).
With DD1 I was taken in for induction at 39Wks as there was literally no fluid left, and endured a week of people who'd just had their babies (big communal ward) comiserating about pre-eclampsia, poor thing. I gave up explaining I was full term.
With DD2 I was literally huge at 6wks, and stayed that way right up until she was 2 weeks late. Many comments at the school gates "Still here then?" is the worst, as yes, I clearly still was, thanks for pointing that out.
DD3 was a nice 'neat' bump. Didn't get much except the 'are you hoping for a boy?' comments. Brought smartly to a halt by, "No, she was unplanned, and I've just split up with my partner, so am a bit weepy just now"...
And now I'm 38+4 with a DS. Two weeks ago I was getting 'gosh, I didn't even know you were pregnant' comments, now it looks like I've got a football up my shirt. The 'did you keep trying for a boy' comments are ghastly. We've moved since DD3, and very few people here know that DP isn't my DD's dad. Have again, given up trying to explain, and now just smile and say 'that's right'. It totally wrong foots people.
Good luck everyone, bumps are fabulous, regardless of their shape or size!