Is this so?

A small environment will stunt the fish--it's not good for most fish, as they don't have the biology to support the delayed growth. Some can slow their growth in small tanks--a result of reacting to the hormones that build up.

A food that contained hormones (though I can't see any possible benefit for manufacturing it) would work, I just doubt it's available, and agree that it would have long lasting negative effects on the fish. Growth can be semi-controlled by controlling food availability and quality, as well as temperature, but there are limits--delaying the inevitable.

I don't know about the food, but while a small tank might stunt growth in some instances it is not a way to keep a fish small. It's a common myth that a fish won't outgrow a tank. That you can keep a fish in any size tank because it won't grow if the tank is too small. This is one of the biggest myths in the hobby and it makes me cringe when I hear people saying it. Don't worry get that ID shark or the Oscar they won't grow in a small tank. A lot of people have learned the hard way it isn't true, but it's an even harder lesson for the poor fish.