i think smaller yorkies grow for a longer period of time than other yorkies. when milu was 8 months old, she was 2.6 lbs, and people told me that they don't think she will grow anymore, but she did, she is now 20 months old and she is around 3lbs.
i think your yorkie will still grow and put on some more weight, i am not sure how much more though.
milu did stop growing for a while just like your baby, she was 2.6 lbs for a couple of months, but then it was like she grew overnight. (after her spay surgery)