I'm surprised you haven't gotten some more definitive answers on why the bbc crank pin sizing is beneficial. The two that come to mind right off. First, less clearance issues in the block and less grinding at the bottom of the cylinder bores with the smaller rod end. Second, less bearing speed, which equates to less friction and a stronger hydrodynamic oil "wedge". Also, I'll have to check,(or maybe someone can chime in) but if IIRC, the aftermarket cranks with small journals have the counterweights set to the heavy side so that material only has to be removed instead of adding weight to balance them.
