Their cases have been microscopic since 1978, it's like they vacuum-sealed them around the moving parts. They also tried to keep rotation speeds high, and therefore the torque on the gears relatively low, which apparently allows them to get away with smaller components (particularly the clutch). If you look at the primary ratio of a Maico and then compare it to that of a contemporary YZ (especially the older ones) then you'll see there's a very radical difference indeed.