>> >The surface of the cluster is a dangerous place for varroa

> Interesting conjecture...  I can't hardly imagine that a healthy mite would drop through or over a layer of bees without catching hold.

Then why do we see live mites on the bottom board? 

Of course, on top of any cluster, a mite would fall past more bees than on the bottom of a cluster.  In either case, though, mites falling or being brushed off on the bottom fall free.   

Anyhow, it only takes a small influence on average varroa lifespan to reduce the slope of a upward population curve or turn it downwards, and this is only one of the differences between small and large clusters.  

Diameter	Area (A)	Volume (V)  A:V Ratio
1-----------3-----------	1-------------6.0 
2----------13-----------4-------------3.0 
3----------28-----------14-----------2.0 
4----------50-----------34-----------1.5 
5----------79-----------65-----------1.2 
6----------113---------113----------1.0 
7----------154---------180----------0.9 
8----------201---------268----------0.8 
9----------254---------382----------0.7 

Here is a 6" cluster compared to a 17" cluster
		
6----------113---------113----------1.0 
17---------908--------2572---------0.4 

The surface area increases less than half as quickly as the volume.

Geometric effects in scaling are under-apreciated.

Small clusters tend to have less drone brood and for shorter duration as well.

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