As I mentioned in post 16, I believe that because the chamber is tapered wider toward the breech end and the pressure is not high enough to make the brass stick to the chamber wall (necessary to stretch a case head back and cause pressure ring thinning, as happens with high power rifle loads), the whole case backs up in the chamber and is swelled out fatter at the bottom to fill out the taper and any bulge and unsupported chamber might allow. To supply brass for that fattening, the mouth is pulled down shorter. You can see this measuring the cases before and after resizing. Resizing extrudes the brass back forward, but apparently, it is unable to achieve 100% return to the original length (except that the first time you resize a once-fired new case using a carbide die, as the ring eliminates the taper in the case, so that first time it may result in a longer case that grows shorter subsequently).
My expectation is that a wider, looser chamber will cause greater shortening per load cycle than a tighter chamber does. I've never tested it, but I should. I've got an Encore pistol barrel in 45 Auto that I fitted with a strain gauge for pressure measurements, and it is a fairly tight and fully supported chamber, so my expectation is it would reduce that shortening effect.