Logically speaking, the heavier the payload, the more propellant you would need to put it into orbit. If you only need a certain percentage of your maximum fuel capacity to put a payload into orbit, then any amount of fuel over that only adds to the weight of the rocket, and thus increases the fuel requirements to put that extra weight into orbit. In other words, you're only lifting extra fuel into orbit at that point.
This is, however, exactly what they're doing with the first stage because that extra fuel is used to reduce the cost of the overall mission by safely returning that part of the rocket. $ 1st Stage >>> $ extra fuel.