My little boy is larger for "our" breed too, and I HATE it when I get comments about it. I totally understand that you would be upset, in fact I probably would have slugged the woman.
People are just DUMB! I think the little ones are cute, but that doesn't mean our hefty boys aren't cute too! I've been accused of lying about Beau being pure bred just because he is 13 pounds, now that makes me STEAM! I totally understand; just ignore her. If your baby gets as big or bigger than mine, you'll probably be hearing a lot of rude nasty comments like that! She probably doesn't know one thing about yorkies.