From my observation it seems 3 cubic inch is about the break off point from the original ideas they had about chain size .It seems those above ran 3/8" and smaller ran .325 . You get closer to 2 cubic inch and it drops down to 3/8" lo pro/pico .
On the other end those above 6 cubic inch it was .404 but here of late it seems it can go either way ,3/8" or .404 .That would of course depend on the region and preference of the saw operater .