Yorkie Size Myths Maybe that will help you understand. In my opinion a breeder who loves the breed and wants a good blood line and etc.. they will not want their puppies to be teacups bc usually they are not healthy. When A small puppy is born from a litter a good breeder will take extra care and making sure that puppy is eating well and do what it takes to get that puppy to a healthy weight. I just don't think people realize what they do to get their puppies that small.
There is no way of really telling what size your dog will be full grown. I mean their are the growth charts but for me they were off a little. Pixie was supposed to be 5lbs by the charts....lol she has surprised us and is 7 lbs, the Vet says she would like to see her at 6.5 so we are working on it.