The main stream thinking of the time was for the 360 to use a bigger crank journal for use in the trucks as a bigger heavy duty main would handle more stress under the work load of heavy hauling.
While this was largely proven wrong years later, that’s the way they thought back then.
FWIW, the smaller bearings offer less friction as spoken about earlier but it’s not a big HP maker but more of just simply less friction allowing the rotating assembly to spin up faster and requires less oil to protect the bearings.