A diver breathes more air from his tank to fill his lungs when he is deeper than to fill his lungs with the same volume of air when he is shallower. This is explained by Boyle’s Law which states (simply) that the volume and pressure of a gas are inversely proportional. As you double the pressure (by going deeper), you halve the volume – so it takes twice as much air from your tank to fill your lungs.
Stay shallower if you want to use less air on your dive.
This can be seen visually by these selection of videos.
https://www.youtube.com/watch?v=hXnkJNk45Ek
