that may sound hurtful, but in the large scope of things - it is a fact. MANY fish (as well as other creatures on this Earth) are simply "put together" wrong when they are hatched. Within a single spawning you can get fish whose digestive tract is in particularly GOOD working order, whose fins are just right to give them great swimming ability, and whose eyesight, etc. are great. These fish will naturally grow more quickly than their counterparts who may have one or more "handicaps".
All too often these "handicaps" will cause the fish to be "less than" its brother. That is where fish sort out in "quality" - size, health, strength, etc.
A breeder, interested in the best, strongest, and healthiest will not use the "less than" fish in his breeding program. Most people would not choose to raise a sickly, stunted fish - and breeders generally do not have the room to waste on ones that may or may not even grow to adult size - much less a fish that cannot be sold, shown or bred. They are culled.
Nature culls a lot of them - there are distinct die-off periods in a young fish's life. I think these are times when the fish's size finally outshoots it's ability to cope with it physical problems - and it dies.
In places that bring you fish at very cheap prices, the fish are raised for numbers, not quality. Any fish that makes it to a certain size, regardless of physical presentation, is sold. You CAN find ones that will only be 1 inch at several years of age. But that is not necessarily good. It means that the fish has not had good food availability, good water perhaps, or perhaps has immunity issues or is dealing with parasites or disease or genetic flaws.
As so wisely stated here - the PingPong Pearlscale is a shorter fish than the others - but it makes up for it in MASS. It can be only 5 inches long, but carry the mass of a longer, more streamlined fish like a Comet of much great length.
Goldfish are larger tank fish. They get much bigger than people are used to thinking. I guess what I am trying to get to is that you need to just assume that any goldfish you get - if it is healthy and raised well - will get BIG. That is the nature of the creature. If you want a smaller fish, get guppies.

I love 'em
BIG!