I've never heard any breeder, bad or good, that would guarantee an adult weight. That's not to say it didn't happen. I'm sure, like many, you trusted the breeder which is unfortunate and I'm sorry it happened that way for you. I def. wouldn't lump all breeders in the same category because it can offend the breeders that are good. In life, there are good and bad in everything. Unfortunately, it sounds like you got a bad one...sorry!
Anyway, I would have to agree that going to a show breeder for a pet that looks the way you want would be best. You could start a new post in your area if you're still looking. You could go to:
Yorkshire Terrier club of America Also, there is a post on here that pretty much explains it very well:
The Real difference between a show dog and a pet
Hope that helps!!!