My goldies were about the size of the ones you see in the store when I bought them. Man have they put on weight!
Most people don't realized a lot of the fish you see in the store are babies less than a year old. Things like oscars, silver dollars, and goldfish will be almost double that size when they are adults.
Koi get huge. 12 inches is actually on the small side for a koi. My 12 inch one is the youngest. I can expect him to get around 24 inches before he'll stop growing. They all should top out around that size. I think the largest koi on record was close to 4 feet long. They are another fish that is sold as a baby in the story yet can easily double its size in under a year.