I think it is perfectly reasonable to expect your yorkie to conform to the breed standard. Sometimes that doesnt happen. I think most of us love them anyway, big, small, and in between. The size is a personal preference. It matters more to some and less to others. I think we all just want a healthy baby in the end.
You are right about the fact that adult size is very unpredictable. You never know for sure what you are going to get, you just love it so much it doesn't matter anymore.