AHB are in the smaller cell size, but AHB have Varroa.

Grooming/hygienic behavior seems to be the AHB method of Varroa control.
And, I believe it also is one of Apis Cerana methods of control. It is also
the predominant method found when EHB colonies are located or selected for
that are Varroa tolerant. There are other factors that come into play, but
they usually are with different strains of Varroa. From what I have read,
the method common to all races of bees exhibiting varroa control is
grooming/hygienic behavior. That is mainly leg biting but also opening
cells to dispose of pupa with varroa.

This all seem to lead back to Allen's comments about which bee are we
looking at. If it is AHB, then foundation size probably has nothing to do
with Varroa control. If it is not AHB, then is grooming present? If so, it
is probably not cell size but what is common to varroa control in all bees.

But if grooming is not present, then it might be cell size, but it could
also be a host of other variables.

Bill Truesdell
Bath, ME