I believe when the boats are going slower the props are deeper in the water and water is going over the top half of the prop. The prop slings the water in the air off the top, big roost. When the boats are going fast the hull rides higher so the props ride higher. The water level is actually below the height of the propshaft, so only the bottom of the props are in the water, small roost.

Low speed = bigg slip = bigg roostertail.
High speed = low slip = small roostertail.
Better efficiency = small roostertail.