I imagine like everything in aerospace, 'just scaling up' isn't as simple as it is. Drag increases non-linearly as you scale up for one thing, and with something as tricky as the rotors of helicopter, you probably have to do all sorts of nuts stuff to get it to work. For one thing, you can't just make the rotor span larger (with same rotation speed), cause then your increasing the velocity of the rotor tips which could cause all sorts of problems.
