Planes work similarly well at small and big scales. They are more efficient than quadcopters even at small scale. Quadcopters are just much safer and easier to

Planes work similarly well at small and big scales. They are more efficient than quadcopters even at small scale. Quadcopters are just much safer and easier to control. Compare "fall down at random without input" vs "stop without input". As for why quadcopters aren't used at big scale - moment of inertia scales with cube of the propeller radius. And to control a quadcopter you're constantly speeding up and down each propeller. That's a huge loss of energy when your propellers are big enough. So instead at big scales we use very complicated mechanically designs like helicopter. It uses a propeller that rotates at constant speed (so moment of inertia doesn't matter), and instead changes the angle of attack to control the aircraft. BTW moment of inertia is also why nobody in their right mind would build huge mech robots like in sci-fi. Wheeled vehicles move forward without fighting inertia - wheels spin at constant speed and you just add the energy lost to friction. Mechs constantly swing their huge legs back and forth fighting the whole inertia twice with each step. Fine for toys, not fine for big vehicles.