its maybe the easiest way to "increase" the volume. it actually alters the rate that the air inside the box changes temperature. lets use a sealed box as an example-
when your cone is at rest, the pressure inside the box is considered ambient. when the cone goes in, the pressure inside the box goes up. when you pressurize air, the temperature goes up. when the cone goes out, the pressure inside goes down, and temperature goes down. we're not talking about a couple of degrees, its generally a very small temperature change.
but once you add polyfill or fiberglass insulation, or something of that sort, it absorbs and radiates the heat at a different rate than just air. its a slower rate very similar to having more air, just like having a bigger box. it only works because there is so much surface area in proportion to mass, and finer materials tend to work better.
But the problem is it reaches diminishing returns quickly and will only work up to a certain point. Last time I looked into stuffing it was about 1 lb/ft^3