Chip - the fish you got are bait. I know you think that they are cute, and am glad you are giving them a good home. The fancy koi are not for everyone...

If you want bigger bait, warm them and feed them all winter.
We did an experiment like that several years ago. Control group in the pond, subject to cold and reduction in food/feeding during the cold. Heater/eaters in a heated pond and fed heavily all winter.
We got explosive growth in the heater/eaters... but they seemed to be less resistant to disease in the following years than their siblings.