Most saltwater fishes never stop growing. In captivity they can easily outsize their wild relatives. In the wild, predators, resources, afflictions, etc. take their toll and wean the population.
Although the size of the space is constraining, it has been shown that the size of the aquarium isn't as significant as we once thought. The 'small-tank-stunts-their-growth' concept is not true.
What usually happens is the bigger fish dies in the smaller space from space stress that goes unseen by the hobbyist. The space stress doesn't stunt their growth as it takes a toll on their lifespan. The hobbyist thinks the fish just got old and died. Not.
Without stress, for instance, the Copperband Butterflyfish I've seen in captivity have grown to over 15 inches. If an aquarist manages to keep a 3" Rabbitfish, for instance, for over 5 years, it will be not only just 10 inches long, but incredibly wide.
