Genetics. You could have a breeder that breeds within the standard as far as size with parents both in the standard size wise but somewhere back in their ancestory was a bigger yorkie. When you buy from a reputable breeder like a show breeder they are very careful about their lines and although they may get one that ends up bigger now and then for the most part they can accurately tell you with much more certainty how big your adult dog will be because they know what they produce |