I often comment if a woman's belly looks small and compact. Only because I am so jealous. I looked like a beached whale when I was preg. People thought I was due when I had months I go. A friend looked like she had jut eaten a large meal the day before he had her baby and popped out a healthy 7lb baby. We are all different.
I'm sure your belly is just gorgeous. Don't worry about the comments, it doesn't change your belly or your baby , it's just how you are. :-)