No different than a tire. A 10" tire has no more tread on the road than a 20" tire. A pad has more contact area than a roller, particularly when the pad is allowed to rotate so that it matches the profile of the hull. I know the pads on my trailer cause square marks in the bottom paint, roughly the size of the pad, demonstrating that almost the full surface of the pad is making contact.
(Which makes a good case for rollers - ha ha - then there's less wear on the bottom paint) (I'll shut up now)