Goldfish are limited in size by the hormones they release. They do this for instances in the wild where they are trapped in small pools or to inhibit the size of their competitors. But this does not mean keep a goldfish in a small tank or overstock a tank. This is because the hormones that inhibit growth are first released into the water before being absorbed, since they want to try to inhibit competitors over themselves. So unless you never do water changes or do them super infrequently, you remove the hormones from the water. The stunting that most people see means they either somehow found the perfect balance between water changes and hormone level, or they are killing their fish by not doing enough water changes or keeping them stressed out enough to release large amounts of the hormones which can not get affected by water changes, both of which will impact the health of the fish. Goldfish release this hormones in small or crowded tanks because if they were any bigger they would die.
Directly Sources used: https://www.fishkeeper.co.uk/faq/will-my-fish-only-grow-to-the-size-of-the-tank/ https://petkeen.com/do-goldfish-grow-to-the-size-of-their-tank/ Sources referenced: https://cosmosmagazine.com/nature/do-goldfish-really-grow-to-the-size-of-their-tank/?amp=1 https://www.inverse.com/science/why-are-these-fish-so-big/amp