Wing volume loading does a good job for the same reason that wing loading does a good job.

They both give you a number that you then apply to the list that tells you what the number means. TELLS YOU WHAT THE NUMBER MEANS...... The number actually doesn't mean anything, it's the subjective interpretation that matters. Since both systems rely on a subjective interpretation, who cares which one you use. And it matters little what the number is that gets you the prediction that says the loading is light, medium, or heavy. We're after the word after all, not the number.