She could just be a smaller kitty. My Samantha is the smallest I have weighing in at 8#. Chester who is just a few months younger weighs in at a whooping 17#, but his bone structure is totally different than hers. He is bigger boned, taller and longer.

As with humans, the family genes decide which are big and which are small. Samantha plays a lot and hardly naps during the day and is far more entergetic than the rest. She jumps high, runs a lot and is just sooooo playful! Maybe that is how your baby will be and there is nothing wrong with it. As long as the vet isn't worried, there is no need for you to be either.