We're making new crankshafts for new 9 cylinder F-M opposed engines right now...We make cranks for GE, EMD, and ALCO locomotives & power units, and sometimes see the F-M's.
The F-M cranks are so much different than the normal stuff..they look like they were designed by an artist rather than an engineer. Or maybe an engineer with artistic flair. They seem to be not as "stout" as the other cranksafts; more "elegant" in their design.
In a world where many things are over-designed, perhaps these were optimized for best use of material? Hard to say; the original design is from the 1950's.