[coderenegade]

Quadrotors use rpm-based control, which is simple and effective, but doesn't scale as the rotor gets bigger and the moment of inertia increases. You could switch to pitch-based control, but then you lose the simplicity, which is a big part of the appeal. The other issue is that the efficiency of any propulsion system is proportional to the disk area. Four smaller propellers have the advantage of zero net torque on the airframe, but carry a substantial penalty in terms of efficiency, which cuts into performance.

Fixing these issues inevitably leads you to a helicopter.