Very good question. I think it's really easy to overfeed (or spoil rotten) these little dogs so a lot of them do get a little chubby. We don't have that problem here (yet) but I can see how it developes. Shelby is miss picky so I doll up her food to get her to eat, then she wants more of the good stuff and less of the food. It's hard knowing what to do.
I read (somewhere, since I read everything about Yorkies) that you shouldn't judge overweight by belly. Judge it by putting your thumb and fingers on their shoulders. If it is a V-shape, they are fine...if you get more of a U-shape it is diet time.