It's a misconception that fish only grow to fit their enclosure. People assumed goldfish would because they'd be kept in bowls and happen to die young/small due to improper husbandry. They can live for years- I've heard of an individual koi that's over 200 years old (the typical lifespan is ~40 years for koi and ~25 for goldfish though), yet people assume goldfish have the same lifespan as mice.
Sorry pigfat, I wasn't trying to "school" you or anything; you seem like you know what you're doing and can provide proper care for them. I just didn't want readers unfamiliar with fish to continue believing the misconception if I could help it. And then I geeked out and got carried away. :)