I don't really know, but depending on is age, he may still have growing to do. I once knew a cat, belonged to a neighbor, who's head seemed too small for his body. His name was Hooligan, but we called him Pinhead, that is until he finished growing. Eventually his head grew into proportion with his body.
That being said, my Lickorish has slender limbs and dainty paws (until she unsheaths her claws, that is), while Squeek's limbs are stocky. It may be just a genetic thing, like some people are tall and slender while others are stocky. Neither one of mine is a jumping, somersaulting, climbing kitty, but they do jump up and down from my desk and the bed with no problems. If you're really concerned, you could ask a vet. There could be issues with nutrition and bone density, but I'm not an expert by any means.