think of it like this. there are different types of sizes of fresh water ponds, lakes, lagoons, what have you..So fresh water fish have the size thing built into thier genes..
Ever see a small Ocean??
No, it's not even in thier DNA to worry about size....They usually will grow to thier normal size regardless of what size tank they are in..
it's just none of us can keep these guys till they are full grown, most fish can live 15-70yrs....yes 70!
So chances of them living to they are fully grown is very little.