I don't think you're being over-sensitive at all. You could add on to the "I'm sorry" and say "Oh, I'm sorry... sorry about the vet bills you will most likely encounter!" because that's something that many people who seek out a "tiny/teacup/purse" dog don't have any clue about. My Vet has rescued several itty bitty dogs from mills/pet stores and the health issues are insane!
I just got my Yorkie, and she's a hearty 11.8 lbs! Lol, and then my other dogs are 105 & 80 lbs. I personally like bigger framed dogs of all the breeds that I'm interested in. My one dog who's 105 lbs sat in the shelter for over three months because he was "too big!" people didn't trust him with their kids and felt he would be a nusance in their home! He is the most amazing dog on this earth (in Mommy's eyes of course
) and he's so gentle with kids/smaller animals, and perfect in the house. I can't imagine turning him down because of his size!
A very close relative has a 3 lb pom, and bought her for her size... and while it's a cute dog I don't envy her one bit. It's such an ignorant mentality to shop for that, and any time this dog get's sick it's life threatening. This dog will never be allowed to be a dog because she's treated like a human premie baby. I feel bad for the dogs that are like that, because at the end of the day they are still dogs and deserve the chance to be just that. I don't think dogs were ever meant to be bred down for such small sizes, it's just not healthy.