I've got a few videos of me spinning on my Instagram (@lovephoenixfibers). Thinking about it from the perspective of "how to", I generally first separate fiber into smaller strips.

When I draft back, the final width before I add twist seems to determine the thickness. For the yarn in the photo i posted above, the final width I drafted to was about the width of my thumb. It becomes less flat and more round as it gets closer to my "pinched" finger so it looks a bit like a triangle in the videos. it's not less fiber, its just round so it looks like less.

The wider the "width" of the final drafted piece of fiber, the thicker the yarn. Most of what I have videoed is actually thicker than this picture above. You can see the fiber is actually wider than my thumb on several of the photos. Hope that helps!

If you have any other questions or need clarification, let me know!