First of all it wasn't just one fish, I've seen this occur in many fish species over the years. a fish in captivity doesn't need the same amount of food as a wild fish. a fish will over eat if given the chance, in the wild they have to eat at every opportunity. In captivity you have control of how much they eat. I've seen this effect in many other fish and three other sharks. The size of a fish is pliable, more food means a bigger fish, less food and the fish will adjust to the food supply by being smaller. This happens in the wild as well. This was the first dwarfed iridescent shark i had kept long term, he was a great fish with lots of personality. His life was cut short by a water bird one summer day. My neighbor told me when I came home from work "there was the most beautiful bird in one of your ponds today, I think he was drinking he kept putting his head in the water!" Of course the bird was eating my fish, but she didn't know that. yes you can grow a fish huge in captivity but it isn't necessary to do this, fish can be small and still enjoy long healthy lives. Over feeding can result in a huge fish that lives a short life due to a build up of fats due to their sedentary life style. If you don't want a paddlefish don't buy one. I don't see any reason that runts and culls from the aquaculture business shouldn't be diverted from the land fill to the petstore.
Many people thinks these tankbusters and paddlefish should not be sold in pet stores because 3/4 of them fell in wrong hands. It is very rare for any tankbusters have second or third home. Im sure lots of people disagree with your theory. Do you just based it from your ID shark and other pets you had in past? I've seen too many tankbuster stories and even with their controlled feeding, they will keeping growing.
Glad that this beautiful bird visiting your pond