As you stated, larger pin more bearing area. The larger rollers have more surface area which spreads the load over a larger contact area on the pin and the races. Also the larger rollers turn slower and are exposed to a larger volume of oil which keeps things cooler which makes the oils job a lot easier by giving it more load surface to work on. 5/16 rollers slower speed is also easier on the cages which have larger surfaces where they contact the rollers.
But then HD has gone to a 2" pin with 1/4" rollers on the XR but I believe that was to get a more rigid crank assembly as they made their way to 100HP.
What do you mean by change direction?
I'm not sure what the factory was thinking of when they went to the big rod set.
1. no length difference, so it has no effect on breathing, etc.
2. no change in taper size, so it doesn't affect lock-up, rigidity, etc.
3. the larger big end obviously adds to the rotating weight of the wheels, so (if the intent was to maintain inertia at the 1954 level) weight would have been removed from the halves
4. the small end looks considerably heavier than the previous 1953-54 KR pair (WR without the lightening holes), and even than the K/KH pair, so vibration may have been worse.
Perhaps they didn't yet have any confidence in the 1954 pin & 1952-54 rods (K, KH?)?
I would assume they intended the larger bearing area to be an advantage for long races (were they experiencing failures with the 1939-54 54 × 3/16" rollers?), but I don't think their planning was on target - the new 51 × 5/16" rollers only give slightly more effective area, and have much higher inertia both as the the weight of each roller in the cages, and of the bearing assembly in the rods. This tends to make the roller refuse to change direction and skid - very bad.
More than 1 engineer suggested to me that the obvious choice would be the same rod (if that's what they wanted, or to save re-tooling), but with a larger 1-3/4" pin, and 72 of the old 3/16" rollers for 41% more area.