Because the energy lost to the movement of the propellant goes up as the square of the velocity but the thrust goes up linearly. Bulking out your propellant by

Because the energy lost to the movement of the propellant goes up as the square of the velocity but the thrust goes up linearly. Bulking out your propellant by a factor of ten while reducing the exhaust velocity by a factor of ten doesn't change the resulting thrust, but reduces the energy you need for that thrust by a factor of 10. The above elides a lot of complexity but the Wikipedia summary of the situation is pretty good. https://en.wikipedia.org/wiki/Rocket#Energy_efficiency https://en.wikipedia.org/wiki/Rocket#Energy_efficiency