This is actually quite common. A goldfish will keep growing according to how much room it has to swim. If you release these fish into the wild, they can get up to several pounds, depending on the species. Me and a buddy were fishing at a creek a few miles from my house. I was sitting halfway up the bank, when I noticed several bright orange fish swimming in the water, near the surface. It was a school of goldfish and they were huge. The biggest was probably 3 lbs. My guess was that someone was using them for catfish bait (a common practice), had some left over and just released them into the creek. We tried to catch them, but they wouldn't touch our baits.

---