With a higher oxygen-percentage, you can either get more power from the same size engine, or you can build smaller engines for the same power output.
As imtaar explained in detail, the higher oxygen percentage means you can burn more fuel for any given volume of air. The size of any of these engines is determined by the size of the combustion chamber, which means that for the same amount of power, you can get by with smaller combustion chambers, and thus lighter engines.
Since engines themselves are a substantial part of the weight of every vehicle, especially every airborne vehicle, you get a lower overall weight and in return an even smaller power demand, again reducing the size (and thus weight) of your engine.
Another substantial part of the weight of the vehicle is the fuel.
A higher oxygen percentage does not reduce the amount of fuel you need to carry. for every given engine, the power output is directly linked to the amount of carbon atoms you feed to the engine, so baring efficiency gains that might or might not result from the size reduction, your fuel consumption would stay the same.