It is said to be fully implemented, so the weight and volume caps were put so intentionally, that you can store those items in the backpack, that have weigh more and are by volume larger than the backpack itself?
If you're trying to store a backpack somewhere, you would probably smash it down or roll it up or something so it doesn't take up much room. Plus, my backpack IRL weighs maybe a pound, but it can carry what feels like a ton of books in it.
Yes, the backpack and the books will weight more than just the backpack :) Obviously, though that maybe a little too complicated for Combine, or how do they call it? "KISS"?