Hey this time it wasnt me who brought Mega Man into the discussion!

Command Mission however sort of inchest towards the idea of technological progress being a slight factor in the slender designs. the Artist thought that since Command Mission was 22XX, that it was in the Zero timeline, so he gave X those slimmer legs. (and look at Marino too, for example)
ZX furthers the concept. the more technology advances, the more Human looking Reploids become. by ZX, the more ROBOTIC the Humans become, merging the two into one race.