I used to think the same as you but it's not exactly what you think. The front motor is a bit smaller than rear so at cruising speeds on the highway, the rear motor disengages and the front can handle the speed while using less KW even with the added weight of the extra motor.
That has been true in the past with the S & X, but the motors on the 3 (and presumably Y) are a bit different. The RWD 3 was slightly more efficient than the AWD.
