I don't understand the question.
If it is why do they need such a big box then the answer is because they do. Imagine you are trying to breath out of a freezer bag full of air, then try it with a black bin bag and see which works best.
I am not suggesting you actually try and suffocate yourself, just imagine what would happen.
You would be better off with 1 x 15 or at the very most 2.