I have been to many chinese fish shops and I know the locations of all the fish places in Manhattan chinatown, so I know what you are talking about. The reason is that the fish are imported from fish breeders. Most fish breeders tend to give their fish plently of space and a good, the water also has to be good. In summary a fish's growth is determined by the size of the tank (the bigger the better), the fish diet's (a variety of food is beneficial), and the quality of the water (the water has to have no ammonia or nitrite and it has to be clean.) Most chinese pet shops stock large fish anyway.