I thought Ruger would stop at 6 lbs, but he's over 9 lbs now.  I've heard many Yorkies turn out to be surprises, and there are no guarantees even if your Yorkie has papers a mile long and his parents are small.
 
I like Ruger's size, and the only downside to owning a larger Yorkie is they are very active and curious and can get into more things.  I have to keep moving things higher and higher. 
