5 to 7 lbs doesn't sound over weight to me either...yorkies come in all sizes. I have 3 that range from 4lbs to 6 1/2 lbs, there are some on here that are 20lbs!! (they belong to the teapot club!
) But, as someone here said...if you vet thinks they are overweight maybe they could suggest a good "lighter" food for them....do you bring them for walks? are they couch potatoes? Maybe more exercise would help.