Recent models have aluminium axles. These are wider diameter than steel and are stiffer and lighter BUT something has to get smaller to accomodate the wider axle. They use smaller bearings. They may use thinner freehub bodies but I cant find any data on that, only reports of bursting freehubs that never used to happen.
I cant tell you the Shimano part no of the alu-axle versions or when they were introduced but it was a few years ago. Basically the current XT is not the same hub as the old XT.