i think you can sometimes be lucky with what you get.
We bought Buddy when he was 6 weeks old, he has an underbite and weighs 14lb (both his parents were big aswell) he is 2 and we have been lucky that he has no problems but since buying Buddy and joining YT i know we bought from a 'hobby' breeder or byb.
I always thought 'standard' yorkies were the bigger ones, all the others were toy/miniature ones
Maybe buying aYorkie thats gonna be bigger like Buddy you can 'get away with' buying at 6 weeks but having said that, we had trouble getting him to eat when we got him, then realised he hadn't even got his teeth yet!
If i knew then what ive learned from here now i would do things VERY differently but i wouldnt swop Buddy for the world. to me he is perfect, but i do think we have been very very lucky with how he has turned out.