How does a wave push a board along? Because the energy of the swell is below the surface and the back half of the board is in the wave most of the time, I’m guessing the foil and shape of the tail has a lot to do with how much energy is imparted? So a small wave board with a really thick (and wide) tail may not be as fast as one with an identical plan shape and rocker but with a thin tail despite having more bouyancy under the back foot?
Gravity. You’re falling down the face of the wave. Any part of the board that slows you down helps to control the fall. Wide tail = less board penetrating the water, thus a faster fall.