Tank size conundrum

So if the statement "fish only grow to the size of their tanks" is untrue, what about stunted fish? Don't they technically only grow to "fit the tank" ?

I work at a petstore and it is difficult to explain that to customers...

I would guess that it is a combination of factors including tank size and water quality that keeps a goldfish small(stunted!) when kept in a ten gallon its whole life. (Is this really what stunts fish kept in small quarters?) But this does contribute to the misconception that people have that their 3 goldfish are very "happy" in their ten gallon because they have only reached 2-3 inches long in a year.

>_>

what to do, what to do...

Some people think that a goldfish living 3-5 years is a long life for them, so its OK to keep them in a ten or twenty gallon because thats how long the picture tag beneath the tank says they should live... I have had goldfish for only 5 years now and my 2 are still with me. I tell folks that goldfish, when kept properly, should be living well beyond 5 years... How long on average do goldfish live???

I know the world record holding goldfish was 43 years. But the average is about 15? That's for fancys right? What about shubunkins and comets? my 2 goldfish are shubunkins btw.