My mom has a koi and a goldfish, and we are actually getting a 250 gallon tank for the Koi later this week. The goldfish is going to stay in the 80 gallon. He is just shy of 15 inches. The koi is probably closer to 20 inches. Now, I am not a goldfish person, they just are not my cup of tea, but after you have seen them be properly cared for in large tanks, you would never dream of putting one in anything smaller than a 50 gallon tank (growing them in 20 gallons is okay, but long term they need so much more). Obviously most of them will never reach this potential due to poor care, just like most bettas will be stuck in tanks too small with dirty water.
The one inch of fish per gallon rule only really works for smaller tropical fish in a community tank. Fish that are over 7 inches need new rules.