I know I'm probably over worrying and several people have posted similar worrys, but I need reassurance right now!

Tia went in for her 16 week appointment and is only 1.5 pounds. I always thought that Bailey was small, but she was 3 pounds at the same age. The vet said he was happy because she is eating and gaining. She plays, runs cuddles- everything you would want from a puppy, but she is soooo darn small. I don't need to be worried, right?