the more you carry, the heavier the aircraft, therefore the more fuel used.
Yes, of course, but if you quantify that I wonder what the additional cost is? On a longhaul sector is one thing; what about short haul?
To quote an example; how much additional fuel is burnt to carry the fuel needed for a 20 mins hold at FL100 by a B777 operating a 3,500nm sector into LHR? And how much by a B737 on a 850nm sector into LHR? Does anyone know the answer?
If that 20 mins extra fuel means losing revenue payload then of course there's a real cost IF revenue is actually lost as a result (ie operating at 100% LF), but how often is that the case?