The old line about excess baggage costing more to carry is a bit of a white lie too.
Sure if
ALL pax had excess baggage then yes more fuel would be required to carry it all, however on 90% of flights that isn't the case (excuse the pun). Those that have little or no bags normally off set the ones that do have excess - but the airlines will not let you pool your baggage with total strangers, as they would not be able to make more money out of you then.
Most short haul flights - especially on the lo-co's don't have that much cargo, if any either so the a/c is normally nowhere near close to any of it's max weights - so all that weight it
could carry is going spare anyway.
There are exceptions to this though where flights are weight restricted etc., but the passenger at check-in would never know what flights were or were not weight restricted, so the excess bagagge rule applies across the board.
