Tweaker,
If you think about it, it really makes sense to increase the main bearing diameter, (hence, the torque capacity) as you progress from the front cylinders to the rear ones. Each main bearing journal has to transmit the accumulated torque from the cylinders in front of it, through to the rear of the crankshaft. As you progress towards the output end of the crank, (flywheel end), more torque has been created, thus the need for a stronger shaft. Back when these engines were designed, such good, conservative, engineering practices were considered quite normal.
