2.5% is a gross overestimate. He's assuming that an increase in weight causes a 1-to-1 increase in fuel usage. A lot of that fuel is used to push against air resistance, which is not affected at all by the weight of the plane. The only things the plane needs with more weight is slightly more fuel to reach altitude, and slightly more fuel to maintain lift. The issue on airlines is space, not weight. It's why shipping by air is so much more profitable than transporting passengers. Humans don't weigh that much, but we demand a lot of space for comfort and to store our stuff. The planes can carry far, far more weight than they normally lift. After all, they can fly with only 1 working engine.