The relationship is a bit more complex than that. As a highly simplified comparison, consider that both situations require the same thrust to support the aircraft. At the higher elevation, with lower air density, the props are turning faster and therefore moving the air faster. Since force is rate of change of momentum, that means that the required thrust is produced by a lower mass flux at higher speed. In other words, the product v.dm/dt is the same. But since energy goes with the mass and the square of the speed, at higher elevation, ½.v².dm/dt will be larger.