The whole point of the ion drive is to minimize propellant. Rather than use a chemical reaction to generate energy, power from a nuclear reactor is used to run a particle accellerator. Conservation of momentum says:
MOVO=M1V1+M2V2
So by giving a tiny mass an immense negative velocity, you force the left-over mass to gain a small increase in velocity, whereas chemical systems give a large mass a small negative velocity to do the same.

Think of it this way: the ion drive is like firing a gun compared to rockets being like pushing a bowling ball away from you. Both will probably give you about the same ammount of propultion, but you can carry 100 bullets or 100 bowling balls (you also have to propel the un-used portion untill you use it, which reduces acceleration...).