>I don't understand. If a bee makes a cell size based on it's own body, why would a bee ever regress? If you were born out of a 5.4mm cell, why wouldn't you build a 5.4 mm cell, if that's what your body measurements are?

Body size is a major part. The other part is there is a "natural size" built in to their genetic code. So they tend to get slightly smaller each turnover of comb.