Somebody's probably already pointed this out, but in the case of motors, making them lighter can make a big difference.

For example, by making the flywheel in a clutch lighter, you reduce the amount of torque it takes to spin the flywheel. Saving 10 pounds there is not a 10/3000lb difference.. it could be a huge percentage of total power output.

To be precise, the impact of mass inside the rotor of the motor is 2 * the mass * the rotor diameter / the wheel diameter * the drivetrain gear ratio.

For a typical EV, I think that works out to a factor of around 2.