I'm not sure what you mean by "volume against weight". As I understand it, volume and weight are different measurements -- the former is an indicator of space occupied, and the latter is a measure of mass. So I think you are actually mixing up two possible explanations:
(1) VOLUME (e.g., if the milk weighs 10 oz, then .1 oz is fat)
(2) WEIGHT (e.g., if the milk is 10 cubic inches, then .1 cubic inch is fat)
MrX seems to think it is weight based (2), but I am skeptical. Most liquids are measured by volume, not weight, so I would guess it's a measure of volume (1).