It has been my understanding for more years than I care to mention but in any body of water, aquarium or pond, it is the surface area that matters when it comes to DO in the water.

If you have two 100 gal tanks filled with water only (no fish), (over simplification here) one that is extremely tall with a surface area of 2' X 2' and one that has a surface area of 4' X 4', the 4 footer will have more DO of the the two as it has more surface area coming into contact with atmospheric pressure.

Now to the purpose of the airstone in this equation. It has always been my understanding that there was actually only a minute amount of DO is produced from the bubbles rising through the water.

The real purpose of an airstone was basically to agitate the surface area of the water (create surface movement) thereby increasing the amount of water that actually made contact with atmospheric pressure thereby increasing the DO in the water.