I had one hive that had a very small cluster and I knew they were in trouble going into the storm. They had literally two dozen bees or less. With a small cluster, a hive cannot keep warm and simply freezes solid.
The dead hive was purchased as a five frame nuc from Don Kuchenmeister the "FatBeeMan" in Georgia. I've come to the conclusion that while his bees are pretty good, they just aren't as able to handle the winters here. That's why I'm attempting to add some colder weather stock to my collection this spring.
Healthy sized clusters, probably that of a softball or larger, are able to survive by huddling tightly together and keep warm by vibrating their wing muscles. If they are making enough heat, they can move about the hive as a cluster to find stored honey and keep fed.
Also, if the cluster is not large enough, they cannot keep warm enough to move and may starve.
On the other hand, large clusters need a lot of honey to maintain operations. So it is imperative that they have enough honey to survive the winter.
Thus far, I have forgone a late season harvest so that the hives have enough honey to last the winter. It's part of the more biological/natural/organic method of beekeeping. Because I still have not been able to produce enough honey to keep an emergency store for bee feeding, I still need to feed sugar syrup from time to time, but I always put food coloring in it so that I know not to harvest it as honey. However, as I only feed when necessary, the sugar syrup is concentrated around the brood nest and never in the supers, I have never actually seen dyed honey when harvesting.