In my little bee yard, a small cluster normally means too many bees have succumbed to varroa and flown off to die (long before now) leaving too few bees to keep warm in the winter. There is no doubt that a small cluster WILL freeze out in cold temps. Cold kills, especially when the ball becomes too small.
I have saved colonies that have been reduced to fist sized (due to varroa) by adding electric heat in the winter. The electric heats prevents the cold from killing the cluster off, and since there isn’t going to be much varroa left (they die out without brood), sometimes the colonies roar back in life in surprising size by mid spring.