There is no such thing as a 'miniature labrador'. Did you buy her as a purebred Kennel Club registered Labrador Retriever?
If not she could be crossed with anything, or she could just be from a very small bitch, and if they used a small dog then the resulting puppies would take after their parents.
Have you spoken to the breeders, or seen any of the brothers and sisters?
Without wishing to suggest you have not reared her correctly, it is sometimes the case that dogs that are undernourished as youngsters do not fulfill their growing potential. Sometimes a lack of calcium prevents them from developing good bone. Is she solidly built but not very tall, or is she very finely built?