Re: Typical short-sighted approach
If you add, lets say half a kilo (ounces are only used for buying drugs imo) more plutonium, then you need to add a little more hydrazine for the thrusters to counteract the extra mass.
But then your spacecraft masses a bit more, so the fourth stage needs to be slightly more beefy to get it into the correct orbit, then the third stage has to carry the extra mass of the plutonium, and the extra hydrazine, and the extra fuel in the upper stage, so it needs more fuel, and also extra fuel to lift that fuel...
And so on until your extra half kilo of Pu ends up requiring an extra half ton of fuel in the first stage. The rocket equation is a pain in the bum.