They stop growing. What you have is a pair of fish which need alot more space; most folks think goldfish are small because they manage to kill them off by putting them in the "goldfish bowl," and never really see their potential growth!! Read up on the fish, find out what the growth size is, space needs, etc--it'll surprise you. Look at those pretty Koi in the aquarium section--wouldn't a couple be nice in a huge ceramic planter on the back deck? Those guys get HUGE--over 2 feet; 24 inches. My dad has a pond full.
Example: If you live in a small apartment or house, without any places to go on long walks or run around for exercise, to have a large, active dog like a Dalmation, German Shepherd, Irish Setter, Labrador Retriever (you get the idea), soon you're going to have a very unhappy critter on your hands. Instead of stunted growth, you have "acting out" behaviors such as chewing your most expensive shoes and clothes--or favorites--they always know this. Chewing furniture, bedding, cords, pooping or peeing in the house, barking, inappropriate behaviors--a dog can show us its unhappiness much more in our faces than fish. They tend to just die.
So please, be an intelligent, caring human being and research your fish before you bring them home--fish have brains, however small, they feel physical pain, and they can become quite attached to their keepers.
That's my 2 pesetas worth.