This has been mentioned before in other threads, but it is all about the speed of the blade (-tip), so a smaller diameter rotor will turn faster than a larger. I think the tip speed is usually somewhere around 400-430KTS. With my manuals at home the number stand to be corrected. The rotor tip airspeed will then of course vary with the helicopters speed through the air x 2.
Two problems with the rotor speed: To slow -> retreating bladestall, to fast -> compression problems due to getting close to speed of sound.
Hmm, when I come to think of it, the rotor rpm is almost always the same no matter the size of helicopter: 100-105%
/2beers