I think they do it to reduce the rotating mass and friction of the crankshaft. Smaller bearings create less loss. The rear bearings have to carry more torque than the front ones because they have more cylinders twisting them. The front one only has to carry cylinders 1 and 2 and the accessories and harmonic vibrations. The second one has to carry cylindsers 1, 2, 3, and 4, and the third and fourth bearing carry all six.
