What effect does Varroa have on roof top colonies?

Two out of the three established colonies in my roof top apiary are big and vigorous. Meanwhile Hive B has stayed small since it was caught as a swarm a year ago. Why?

I've been told that some colonies are smaller than others. The bees like it that way. I'm willing to trust the bees to right-size their colony but I'd like to understand what causes the differences. I wonder whether Varroa and its diseases might be an explanation.

Freedom Hives at Headington Hill Hall now occupied

There are 2 Freedom Hives in the grounds of Headington Hill Hall. One is on a tripod next to the garden; the other is in a small section of woodland. Both are now occupied. I don't know whether the colonies found the hives on their own or were brought there.

The Freedom Hive mimics a log hive. It is a perpetual hive without frames. Human access is through the base but there is limited scope to intervene. In the event of them contracting a serious (notifiable) disease the colony would have to be destroyed. In all other situations they will just be left alone.