I would think the diameter of the journal bearings is defined by the load on them (the area required to sustain an oil film).as small as possible (ecomic and friction motives), as big as needed (oil film).The weight between the combustion proces and the crankshaft (piston,piston rod,...)adds to the load of the bearings (inertia)
just by putting one big piston with a crosshead,all the metal used for the pivotting levers could be used to build the rest of the vehicle.
