4.9mm bees

I was thinking about the regression of the 5.1 mm bees back to the original version 4.9 mm. The Books say bees will naturally construct the 4.9 cell, with no help such as foundation, there is 4.9 mm foundation available. If the bees are 5.1 and construct naturally 4.9 how can they possible reach the larvae and eggs to clean and feed them. Understand after a few generations or so the 4.9 bees will preveil, until then how will the hive function? I am looking for a totally natural mite control system. and the smaller bees seem to occupy more of the cell during pupation, too little space left for the mites, also noted a slightly shorter pupation cycle So how much shorter the 21 days is the 4.9 bees life cycle--the reading doesn't indicate. 4.9 bees are alsdo alledgedly better housekeepers then their larger sisters, apparently better able to grab the mites. How does this affect the disposition of the bees will they revert to a more aggressive demeanor.
Barry