There is no breed called a miniature boxer nor is there a toy boxer, has the vet checked her for every possible medical problem?????
I would get a second opinion on her size/age if he doesn't come up with something soon. Also contact the breeder and start asking!
That seems terribly small for a boxer. Then again I have two males One is 4 years and 45lbs, the other is not even 2 years and 75 pounds, so they do vary in size - however my 45# is an all white deaf, that I adopted from a rescue - and is probably the result of some serious Back yard breeders, my other dog is from a reputable breeder. So I would definately call the breeder and get some info on the other pups. I am not saying not to keep and love and care for your dog, but you should try to find out what you are dealing with so you can be prepaired. And as others pointed out, she could be sick. Good luck and keep us updated