5% more fuel = ~5% more weight, which means you need another 3% to accommodate for that, but wait, that's more weight, etc etc etc. You get diminishing returns

5% more fuel = ~5% more weight, which means you need another 3% to accommodate for that, but wait, that's more weight, etc etc etc. You get diminishing returns adding size/fuel to a rocket. Play Kerbal for a while and you'll see this in action.