The best example to demonstrate this that I think will be familiar to most people is the goldfish in the bowl example.
Yep, they have no idea what they're talking about. Keeping a goldfish in a tiny bowl with no filtration or aeration is not appropriate and will cause ammonia levels to build up ridiculously high. This is what causes the stunted growth, not "the fish cannot move around so it stays small." I've seen Oscars grow to 30cm or more in a <30 gallon tank which had proper filtration and nearly constant water change, and I've also seen them stunted to like 15-20 in tanks that were 100 gallons or more but rarely cleaned. These stunted ones also had major health problems, such as fin rot, HITH, HLLE, etc.
It isn't impossible to stunt a fish's growth (it's actually quite easy) but it is nearly impossible to do so while keeping toxin levels within safe ranges and giving them enough food/oxygen. Basically, don't try it if you care about your fish.
The growth hormone level does play a part in it, but not nearly as much as other things, especially if you have appropriate filtration. Basically, smaller tanks will require much more powerful filtration and more frequent water changes, which will keep the levels consistent with much larger tanks, assuming the water parameters are kept to the same standards.
I have heard about people keeping the water a few degrees cooler than normal to slow the growth (it kinda slows everything...). In theory it'd be okay as long as you still keep it within safe range (their immune system starts going to hell if you set it too cold). If you really feel the need to try this, look up the appropriate temperature range for the species, and set the heater to a degree or two above the lowest temp on there, no lower. This will also reduce the nitrogen levels somewhat, but the water will hold less oxygen, so you will need more powerful aeration. A couple weeks before you put the fish back in the pond, start ramping the temperature up until it matches that of the pond water.