I calculated volume for a polygon with an area of 22.5 hectares using a TIN. I got a very small volume measure of: 4.3061518071E-05
What are the units of volume in the Polygon Volume Tool?
Side note: the TIN was created from an ASTER DEM using a height of 9m to define the plane. I calculated volume below the plane. In the attached image, I'm interested in finding volume for the grey area called "Kanjumarg Cell 1"