The larger diameter Al axle change was intended to reduce the likelihood of bent axles on older frames which could not accommodate newer thru-axle hubs. This happened around the same time MTBs transitioned from 135mm QR to 142/xxx mm thru axle.
Probably no one has ever objected to the larger diameter Al axle. Shimano fit a larger diameter axle into a hub shell that's the same size (otherwise they have to redesign and remarket their freehub, cassette and rotor mount), which forced the use of smaller diameter ball bearings. Smaller ball diameter means a smaller contact patch between bearings and races, or force applied over smaller area on these parts (higher pressure), which increases bearing friction and likely results in faster wear of parts and shorter hub life (which is wheel life in the case of Shimano hubs which lack a replaceable cartridge bearing).
Because bicycle tourists choose long term equipment reliability over a capability to make jumps while bombing down a dirt mountain track, some view the Al-axled rear hubs (small balls) as inferior to Shimano's steel axle/larger ball predecessors.
Edit: FH-M756/756a/525 all use the venerable 10mm steel axle and 1/4" balls.
Last edited by seeker333; 06-11-20 at 12:44 PM.