I have met many plus size yorkies, and I have no reason to think they were mixes. Just big girls and boys.
I have a friend who is looking for a large Yorkie. Her 5 lb. Yorkie, named Teddy, was tragically killed by a Golden Retriever.
It really traumatized her, and she does not want to own a tiny dog again. The larger ones are not so fragile, which can be an advantage.