Dave Cushman  wrote:
>But it does not tell a complete story... 

I think there is another plausible explanation as to why the size of cells
shifted from 5.0 to 5.4 mm. When foundation mills were first made in the US,
they were somewhat crude by today's standards and it was probably easier to
set the size to 5 cells to the inch, rather than at an uneven rate such as 5
to 1.063 inches. Root says: "the best specimens of true worker comb
generally contain five cells within the space of an inch, and therefore this
measure has been adopted for comb foundation." It is true that larger sizes
were tried but if they are too large, the bees make it all into drones.
However I think the slightly larger size is closer to the correct size for
A. mellifera and A. carnica. 

[Cell size of races, from smallest to largest]

4.5 - 5.0
A. m. scutellata
(South Africa)
A. m. lamarckii
A. m. litora
A. m. yemenitica
A. m. adansonii
A. m. scutellata
(Tanzania)
A. m. capensis
A. m. sahariensis

5.0 - 5.5
Africanised
A. m. unicolour
A. m. monticola
A. m. ligustica
A. m. mellifera

5.5+
A. m. carnica

Although 24 distinct taxonomic races of
A. mellifera have been described (Ruttner,
1988) there is only one clear case where a
race of A. mellifera exhibits natural tolerance
towards V. destructor i.e. mite infested
colonies can survive indefinitely without
assistance from beekeepers. This is the
Africanized bee (AHB) a hybrid of
A. m. scutellata from South Africa and
A. mellifera from Europe

mite populations in
AHB fluctuate during the year but their
numbers rarely exceed several thousand
while mite populations in both A. m.
scutellata and A. m. capensis colonies have
been reported to regularly exceed 10000

Although reproduction of Varroa sp. is
affected by the space between the developing
bee and cell wall, reducing cell sizes as
a mite control method will probably fail to
be effective since the bees are likely to respond
by rearing correspondingly smaller
bees which explains the close correlation
between cell and bee size.


Source:

"Reproduction of Varroa destructor in South African honey bees: 
does cell space influence Varroa male survivorship?"
-- Stephen J. MARTIN, Per KRYGER

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