What kind of foundation if any and comb are you using? Natural foundationless comb tends to have smaller cells causing smaller bees, which have faster metabolism and shorter brood cycle time thereby being a natural deterent to varroa mite infestion. This might be a good thing.
Also, what genetic breed or strain of bees are they? some are smaller than others. I have 8 hives here and some hives have noticably smaller beesize than others, even on plastic foundation of the larger cell size.
I've been keeping a few hives for the past 20-25 years, and no longer treatment free as I got tired of the losses.