Whenever someone has commented on how big I am I usually just say "yep, i'm growing a big healthy one!" I think being "big" when your pregnant is good, it means everything is going well!

I really liked the reply Suse came up with too:
To 'gee, you're big': 'gee, you're rude/annoying/obnoxious, but I don't feel the need to call your attention to it, do I?'