Goldfish (carassius auratus), come in many varieties, and yes, some will only grow to a few inches or a bit more, while others will grow to similar sizes to carp, also once in the wild, they will cross breed with other carps. Those little chubby things with the bug eyes and double tails are the same species as some of those big ones swimming around in some waters, kinda like dogs, there are yorkies and great Danes.