I often just model a lump of material to get its mass (too lazy to do the calculation).  This works fine and it is quick.  However, the units used in the document settings mean that the density of the material is not shown because at grams/mm^3 there are too many decimal places.  It just shows 0.00.  See image.

If I set the units to length - mm, mass Kg and volume m^3, it shows a sensible density and mass but the claimed volume is 900,000Kg and the surface area is 309,600 square metres.  See image.

So, a 300mm x 500mm x 6mm thick al alloy plate has a mass of 2.43Kg and a volume of 900,000 cubic metres!  It seems that if you change the mass and volume units SW assumes you used those units to create the part - except that the mass is correct.