When you add fuel, you have to add (exponentially!) more fuel to lift the fuel you're adding. It's true that a 200 lb passenger doesn't cost twice as much as a

When you add fuel, you have to add (exponentially!) more fuel to lift the fuel you're adding. It's true that a 200 lb passenger doesn't cost twice as much as a 100 lb passenger, but it's more than the 17% increase one might expect going from 600 to 700 lb dry weight.