Yes, they certainly grow but at a lower rate than in the warmer months. Goldfish grow faster in the warmer month because they have more access to protein (mosquito larvae, daphnia, blood worms, tubilex etc) and there immune system is stronger. Since there immune system weakens and the slime coat (protective barrier to diseases is weak in the winter months, it is not recommended you feed them. If your pond's water temperature reaches below 10 C, then the goldfish will go into hibernation. In the harshest winters, when the temperature gets below 0 C in my pond, I don't see them swim but hide under the caves I made with large rocks.