what is reason?, when we use 3 cells a smaller prop is recommended than 2 cells. infect 3 cells give higher power 

Bigger Prop = Bigger load (as more Air cut)
More Cell = Higher RPM, more Air cut per second.

The Max power output of a motor is fixed (Max Wattage rating). So you cant increase the Prop Size and Cell Count.
If you can clock max 30 KM/hr on a by cycle alone, can you seat your friend (at back/front) and do the same speed ?

To put technically what Ujjawaana has said...increasing the number of cells increases voltage. Now for a motor Kv (RPM per Volt) is fixed....so when you increase the voltage...the rpm with which the motor rotates increases (as RPM has to increase with increasing volt to keep Kv constant)....now if you dont change your prop to a smaller size...the motor would have to increase the current draw to rotate the prop at higher RPM...and thus will probably burn itself.

you could also refer this thread for detailed info........
http://www.rcindia.org/electric-power/understanding-electric-power-systems/