in your original statement you said "compared to many schooling aquarium fish channel cats are quite sedentary".
Comparing aquarium fish like tetras or barbs to channel catfish is apples to oranges sir. Channel catfish compared to bullhead, blue, white, flathead, or any other catfish species can be considered very active. though they are catfish and can be considered slow when compared to schooling or other fish.
And how can you force a fish to grow to its maximum size? you feed it, it eats. You cannot force food down its gullet and pull on it to stretch it out. A fish is going to grow regardless of food or water quality. It may take longer if one or both of those is not ideal. Eventually the fish will reach it's maximum size. If space is limited the fish will not grow to its full potential. Resulted in a stunted fish. Id call that bad fishkeeping.