I know how you feel! I get that question ALL the time. It really makes me laugh when I have my 6lb yorkie Zoe out and they ask. But I do see ALOT of poorly bred oversized yorkies so I can kindof understand why people have that idea. I try to nicely educate them. Some are open, others you can practically see their eyes glaze over.
Of course, the other question I get that really makes me uncomfortable is when they ask how much I paid....