It's a bit late to answer this but here goes anyway (while my tea is brewing
).
My guess is that BSA decided as that if the journal on the new crank was larger in diameter, hence had more bearing surface, it could then be narrower, being narrower of course reduces the bearing surface.
I've not tried to work out which journal/bearing type has the greater surface area, I doubt there is much difference.
I don't know where I heard it from but the crank design change was initiated due to small journal cranks breaking at the join between journal and web (or at least flexing too much), as opposed to big end failure, if true BSA would have concentrated on strengthening that area rather than making the big end last longer by having greater surface area.
The small journal crank seems to be generally accepted as fit for purpose provided the engine is not highly tuned eg high compression Pistons, 357 cam etc.
Time for tea
