shubunkins can grow at considerably different rates in the same exact environment. I have some that were bought from the same batch at the same time two years ago, some are about 4-5 inches long where one is well over 10 inches, all six were about a inch or so long when I bought them, I also bought one last fall that was about 2 inches when I got it and it's already close to six or seven inches long, weird thing about him is he rarely eats fishfood, he always grazes but he loves english peas and and watermelon. Sarrasas, comets, shubunkins, all are the same species of fish, It is stated that they can reach 2 feet. I've seen pics of some 16 but that's about the biggest that I would actually believe without seeing it with my own eyes, more commonly most goldfish in ponds usually stay under about 12 inches with 15" being pretty darn rare. I'm sure some genetics have some to do with it too, but how many times to you get to buy goldfish that you know what the parents even looked like?