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"
Projected data would be best since I see no mention of the data being projected prior to calculations, hence a small coverage in decimal degrees is being interpreted as meters giving rise to the small volume.
Give it a try... project the input data used to derive the TIN (is it raster or vector???), produce the TIN, then derive the volume would be my workflow