Secondary Explosion Effect

[Big Smoke]

Much like how a gas tank is more explosive with a little bit of gas rather than a full tank (which is the vapors that cause the explosion), one theory is that of oxygen content in the case (fast burning powder mixed with too much oxygen in a contained space). I don't know if this have any validity, but the theory sounds plausible to me.
That's why there are minimum loads, and maximum loads.