Hey N.,
Compared to my son, yours is skinny. :) At 9 weeks old, my son was exclusively breastfed and was 15lbs 14oz and only 24 and 1/4 inches. Now he is a lean (and quickly thinning) and super intelligent 20 month old. You are providing such great and important fat nutrients for your baby's brain. And once he starts toddling around, he will not have time to stop to eat--but you'll have provided a great foundation for him. Yay you! You are a great mom! Keep up the excellent work!
I can totally relate to everything that you mentioned. In fact, a complete stranger shouted to me as he cross the produce section of the grocery and came to tell me how big my son was. My m-i-l told me that I was making my son unhealthy and that I was going to cause him to have a heart attack. And everyone else (and their brother) had a comment about/for me. On occassion, I worried that these people were right. And sometimes their comments kept me up at night. But, in the end, I'm the winner. My son is incredibly healthy and smart. And I am proud of the choices that my husband and I have made for him.
You may know this already, but breastfed babies are typically bigger than formula fed babies. This trend goes on until they are about 9-12 months (I believe). If you haven't considered it already, you might want to go to a breastfeeding support group. The Elizabeth Blackwell Center has a great one as well as some great lactation and parent consultants. There you can feel more "normal" and not feel the need to defend your child's body.
As far as responses go, if you can, try to rest in the knowledge that you are a totally wonderful mother--that you are doing what you know is right for your child. People are going to say things about your child (good and bad), and it hurts, but there is nothing you can do about it. They may or may not change their comments/opinion based on your response. And chances are if they are rude enough to say something, your response isn't really going to change or stop them. For close friends/family, just be honest and tell them that it hurts and ask them not to say anything more. Or you can tell them the pediatrician says it is healthy and normal. Or you can tell them the facts about breastfed babies. Or you can tell them the information about the importance of fat in the development of an infant's brain. Or you can tell them to go fly a kite. hahahaha
Anyway, good luck with mommyhood. Feel free to contact me if you need any other information or if you need a pep talk.