Voltage is responsible for speed while Amps are responsible for torque(might be the wrong word).
Meaning that the higher voltage you have (the bigger series) the higher your max speed is, while the higher your amperage is (bigger parallel) the stronger it is.
Now when you take the motor into account. The ESC can adjust the duty cycle, which in a very inaccurate but simple explanation is that voltage is turned into amperage. The reverse is not possible unless I am wrong.
See Battery Amps, Motor Amps and Duty Cycle... Please Explain for explanations from people who know this better.