Obviously the flotation capacity of a board is based on the displacement characteristics not the volume (unless the board is fully submerged!).
So you could have a board with 5L (yes just 5 litres) that would float 300kg or rider & rig weight ... however the board would have a similar shape to a small aluminium dinghy!
Not quite a realistic comparison for a windsurfing board but the principle remains the same. ie a board with an extra 20L with a big domed top surface could provide the same flotation as a board with 20L less a flat top surface.