oh my to the lot of you....
anyways I think that the size of the tank itself is somewhat irelevent, but water quality and food source is paramount. for example bettas can be kept in very small containers....yes I hear you boo hiss....yea I know, but if that betta kept in a small growout and is very well fed and the water quality kept top notch then yes that betta will still grow to its full potentional.
perhaps the best thing that larger tanks do it make it eaisier to maintaine a perfect water quality with less difficulty.