True I would think it should be larger but doesn't tank size, feeding, water temp and all play a roll. Just to be clear I know that sharks don't grow to the size of the tank. I would think that a larger area would let them swim more freely and grow faster. Plus I would think that the larger volume of water would be benefit the sharks health and growth rate.
I think that the white tip reef that I just got would be a prefect example of a shark that has grown much slower that it should have. After talking to the owners of the LFS it came from they informed me that they have owned that shark for 8 years and it was about 24" when they got it. Now correct me if I am wrong but for a white tip that is a least 8 years old it should have grow more than 18" in that time. I hope the small tank and slow growth on my shark did not take too much time off his life. Anyway I was just putting that out there as to maybe why his bull shark has not grow so fast. I would think the level of care at a public aquarium is much higher than at home so maybe thats why these animals seem to grow so fast and thrive.