I am 37 weeks, and I am told repeatedly, "you are that far? You carry really well". I take that to mean that I look "small". My DD was estimated at 7lbs, but she was born at 5lbs 7oz and 21" long, and perfectly healthy. To this day she is a little wisp of a child. She is what she is.
The reality is that every woman has a different body type, a different way of carrying her babies, and that those weight estimates are grossly off target. They average in at 2lbs either way and when you consider a 7lb baby, we are talking about an estimate that is a staggering 28% off target in either direction! That is more than 1/4th. I feel it is very poorly practiced medicine to tell a woman what your guesstimate of her babies weight is at all. These doctors build their practice and their knowledge base off of research and studies, and then they warn women of a babies size based off of a measurement that would NEVER pass muster in research analysis. Can you imagine your doctor suggesting a medication that failed or caused issues 28% of the time? NO.
Trust your body, and your baby. If you are eating healthy, and taking care of yourself, your baby will be fine. Your body was made for this, your child is thriving, and you are about to meet him/her for the first time face to face. Enjoy that anticipation, and when someone says "you look small" just nod and smile. What do they know? They don't know your body at all.