She is only 14 weeks old? She has alot of time to grow..usually you double the weight at 12 weeks..so if she is around 3 pounds right now..she should reach about 6 pounds as an adult..they all grow at different rates thats for sure. If your vet thinks she is going great then she is..
it wont be long and she will start growing more and more..filling out and getting a bit taller..I wouldnt worry..are your two new yorkies going to be puppies?
Dawn