Coming from motocross and lots of messing with shocks and forks I can actually answer why moving to a larger diameter is smoother.
Think of it like a leaf spring short springs take more force to bend, so do valving shims and the larger valve allows larger shims which also adds more surface area to distribute the fluid, so you can actually run a softer stack and get similar results. Now In SBD's case bypass tubes add more plushness to the areas where the tubes are located.