I may be wrong... but for the most part, koi will grow regardless. If you place 3 koi in a rain barrel... you will soon have 3 very large (probably unwell and very unhappy) large koi in a crowded rain barrel.
Goldfish however will grow to size of pond... BUT... that does not mean that you can overstock. They grow to size of container, not resources. If you place 4 goldfish in a 10 gallon tank, with plenty of O2, food, and filtration...you will have 4 happy goldfish that will still grow so slowly that they will be fine. Move them to a 500 gallon... and they will start growing though. If there is not enough resources, but plenty of room... they will grow... but be very unhappy and unwell anyway
Did that make sense?
In any case... you really cannot overstock and have happy fish. If you have a mini-pond...go with the goldies...the koi WILL outgrow it.
Perhaps someone here can tell me if I am right or wrong.