I totally totally feel for you!!!
I have a small baby too and it gets sooo upsetting. We can't go anywhere without someone asking her age then going, "oh!! She is soo tiny!"
My DD was born small, at 5 lbs 13 oz and at about 2.5 months she was at 9 lbs 9 oz. Her 4 month appt is in another 2 weeks and I'm sure she is over 10 now, but I don't know by how much, I just hope she has gained enough that they can keep all their mouths shut.
I actually had a woman ask me this weekend if my DD was only a few days old!!!! Oh yeah! Cause all fresh newborns can hold their head up and "talk" back to you and smile and laugh.
My DH has finally actually started to get POed about it too and was like, what when she starts walking they will still be asking, "oh is she only a few weeks old?"
I try to let it roll off, but it's so hard to hear. All babies I know are SOOO big.
I love DD being small though because it would hurt more to hold her long periods of time if she was bigger and she got to stay in some clothes longer, not all, because she is sprouting in length considerably! lol
So, basically... you are not alone! It stinks when people can't keep their small baby comments to themselves.