well what you are forgetting is the amp draw savings on 12s versus 10s. More volts equals less amps. You can run a smaller 12s and get the same flight time as the larger 10s setup.
Examples are more extreme on 6s to 12s, but the math holds true.
My E5 on 6s: 5000mAh Thor 65c = 4.5 minutes
My E5 on 12S: (2) 2700mAh 50C Optipower = 6 minutes
I don't know the weights off hand, but that still illustrates the difference in voltage.