Enjoy her. She will grow, she is still too young to tell just how big she will be though this that may give you an idea.
http://yorkiepassion.com/growthchart.htm
I have one friend who had one that completely stopped growing at 6 months and mine is still growing at 11 months. Anything is possible.
Weight doesn't tell much. If a dog is a picky eater and underweight that doesn't mean they are tiny, it means they are skinny
Anyone that is concerned about a dog being underweight should get Natural balance meat roll. Dogs love it and it fattens them up in a hurry.
I have a smaller Yorkie and I was totally comfortable with her size once she made it past 3lbs. She could (and does) everything that bigger dogs do. Goes for walks, plays rough, runs up and down stairs. I don't let her jump on and off furniture (when I can help it) but with the knees this breed has no one should allow jumping.
Enjoy your pup, just keep her safe from big feet.