Well think about it, everything that is in your stem, would be in the cotton (mostly). It doesn't take up ANY room, it's just a very thin layer under the screen, and in fact even kind of "fills in" the holes in the stem (without adversely affecting airflow much). The screen is nearly flat so you basically have your entire chamber to fill. I guess you're right, I suppose some of the oils off the cotton do vaporize, but after many, many bowls, I still have cotton that gives off tonnes of vapour, more than enough for my satisfaction. I will have to save some up though, as I would like to know what the high is like from sober. I've only vaped the cotton after many herb bowls lol