true, RPM would probably go down with a bigger rotor...

but we are more interested in PULSES per minute! So even if the rpm goes down you can still have the same (or more) pulses per minute by adding more magnets to a bigger rotor... you will probably have the same "PPM" as long as the magnets are spaced the same distance from eachother on any sized rotor.

in the end the energy from the rotor is not as important as the energy going into the batteries.

The motor is a bonus!