Your post made me laugh - thank you! (not making fun of you at all, just an "I totally hear ya sister!")
I get so frustrated about this too! The worst place - the Babies R Us mother's room. Talk about a henhouse for the busybody moms who want to do nothing more than compare their baby to yours and them tell you how much more advanced theirs is.
If the person is annoying and invading our personal space, I keep my answers short and curt, like I'm the most boring person on the planet with no personality and keep moving. I am okay with the old grandmas in the grocery store taking a look though, since they pretty much know better than to touch. You could also tell them "oh we need to be careful - baby has a cold".
Here's my favorite - my baby is a big baby (75th percentile in weight...I could care LESS about percentiles, as long as he is following a normal curve). Our pediatrician has assured us that he is healthy and we are by no means overfeeding him. But, people feel the need to tell us all the time that we have a CHUNKY, CHUBBY, cute little FAT baby. Similar to your situation, would anyone go up to an adult and be like "oh my, you're so chunky and fat! you certainly don't miss a meal!"
Want to know what I tell them...."well, at least he will grow out of his baby fat". Yes, I know it is mean and petty but it really makes the person think twice before speaking again.