When you raise the lever in autorotation, the inner boundary between the high AOA or stalled section and the driving section moves outboard. The outer boundary doesn't move much hence the driving section is smaller and the RRPM reduce. The next question that usually gets asked is why do we reduce the RRPM to the lower end of the band to reduce R of D and increase range. The answer is that the overall L/D ratio of the whole rotor system is usually arranged to be better at the lower end of the band, beneficial if you overpitch and regardless of weight, you can always achieve it in autorotation by raising the lever.