"yes a larger prop does make for more efficient thrust generation, the issue is that the majority of the volume of air generated is not over the wing but rather to just generate thrust to move the aerofoil through the air to generate lift. What they are trying to achieve here is different, they are actively moving air over the entirety of the aerofoil, large props isnt going to help because beyond a certain size you are no longer moving air over the aerofoil, better to use an array of smaller props to create a blanket of moving air over the entirety of the aerofoil."