That's correct on the growth hormone. If fish are kept in too small a space, or when water conditions are poor, fish put a hormone into the water that stunts growth. Its whats responsible for stunting goldfish in bowls for instance.
Regular water changes can remove this hormone and help. Ive seen instances where fish that grow large can be kept in very small tanks and grow to full adult size if they are fed a constant supply of fresh water and the hormone is never allowed to build. There are obviously other health and ethical issues involved in this. I dont know this is the case, but i assume extensive fish farming must do something like this.