Hi, Cathryn! Welcome to YT!
As long as you love your pup, does it really matter to you what size she is? I am not a breeder, but I know from visiting here since August that Yorkies not only come in various sizes but many different colors as well, including red! As long as you love her and she loves you, size should not matter, provided she is healthy. Have you had your vet look at her since she has gained weight? Might be a good idea, just in case.
Hey, lots more puppy to love, right?