There is some missing info here. How old are the fish? How big were they and how big are they now? How many inches of fish per gal etc, are they feeder goldfish or something else perhaps? Do you feed them in the summer? Are they all the same size or are some bigger? Do you have plants in the pond, insects etc.?

Just what you wanted right, a lot of questions no answers. ;-)

Althaugh there is some different species that will grow bigger then others, fish in the carp family (gold fish, koi...) will tend to be bigger in bigger pond, bigger in a well oxygenated pond and bigger in a well feed pond. It is the same for salmon and trout; I have several rasing units in different locations, and depending on pond size and condition the fish from a same spond will usually grow bigget faster in the large oxygenated pond with automatic feeding then they would in the smaller cirular ponds.

Fish grow to the size they are meant to be, but there are certain factors that will inhibit that growth. Lack of food, genetic defects, over crowding. Feeder goldfish often are defective and raised in over crowded tanks which inhibit their growth. Fish also emit a hormone that inhbits growth in other fish which is why you often get one that is much bigger than the others. It may be something that happened before you got them or if the pond is overcrowded it may be the fish inhibiting themselves. Regular water changes may help that. Or they may be as big as they are going to get. Growth is often slow so you may not notice the increase, most fish grow quickly for the first couple years then it is slow but steady after.