I sware my dog has midget syndrome or something. I'm serious. Is that just genes or why is she so small? help?

I have a purebreed labrador retriver, and she looks like a purebreed normal lab. But I don't understand why she is 2 years old and hasn't even grown to 2 feet. Shes healthy and acts like a normal dog, but it that her genes or does she have a miget disease for dogs? lol. She had puppies, and they grew to be normal size. They are bigger than her,but she is way older. And the puppies were all normal size,so why isn't she? The vet said she's healthy but I'm just wondering if thats normal for a dog? And its not like shes a couple inches smaller than normal,shes a couple feet smaller than normal.