I've been told to expect yorkies to be fully grown at 6 months and to go ahead and switch their food to adult food then, actually. I read it on this forum, there's a thread about it and that's what made me realize that I needed to switch Sully to adult food b/c there's too much unneeded fat content in puppy food once your dog is full grown.
Here's a thread about that:
http://www.yorkietalk.com/forums/showthread.php?t=4054