Bettas usually get a couple inches long not including the fins. The crowntail is probably younger. I have a HM who is still smaller than my other males and I've had him for 4 months. It probably depends on how big the parents are too. I wouldn't worry, even if he is a little bit smaller, it shouldn't hurt him unless he's VERY small, than it could be a birth defect. Could you take a picture of both of them so we can see the size difference?