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"
What are the coordinate units in? meters or decimal degrees?
Angular unit is degree for the TIN (WGS-1984)
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