Grainger has it exactly right. If the disk is small for the weight of the machine (high disk loading) the power needed to hover goes up. That is why a tilt rotor needs about 40% more power than a helicopter to do its job, its rotors are proportionately smaller and more highly loaded.. .. .Down wash velocity is also part of the problem. As disk loading increases, the speed of the air that supports the helicopter must go up, and downwash velocity increases. Big helicopters do not make higher downwash velocities because they are bigger, it is disk loading that makes the difference.