The motor always ATTEMPTS to turn at a speed of Kv x Volts...it never actually makes it because of losses. The bigger the prop and/or higher the current the more the losses and the lower the actual speed attained.
If you use the same POWER (i.e. watts, which = volts x amps) then you'll find that with a higher voltage you need to use a much smaller prop to keep the current down. So whether 100W turning a 6" prop at say 13000 rpm on 3S or an 9" prop at maybe 7500rpm on 2S works best on a particular plane involves so many variables that I'm not even going to try to sort them out.