It takes some time to figure out how thin to draft (how much to feed) to get a given thickness of yarn. Take a mental picture of how much you are drafting (or heck, save a real sample if that helps) and compare that to the thickness of the yarn. Yarn too thick? Draft it out thinner. Take a new mental picture/sample and repeat, until yarn is the thickness you want.
On the reverse -- if the yarn is too thin, you need to look at how thin your drafting triangle is, and draft thicker, get more fiber in the drafting triangle so the resulting yarn is thicker.
You control how much is in the drafting triangle by how thinly you pull it out. You can have very little fiber there for a very fine yarn, or a whole bunch for a thick yarn. I usually start people out with a "pinky-thick" guideline for their first spinnings, which is a chunky yarn, and then have them draft thinner from there once they get the hang of it.
If you're having trouble pulling the yarn out -- move your hands further apart. The best advice I got when I started to spin was to check the staple length (length of individual fibers) of what I was spinning and to keep my hands 1.5 * the staple length apart (staple-length-and-a-half apart). Then the fibers slide easily in the drafting triangle -- well, once you keep all the twist out of it, too. That was key advice #2, LOL.
(posted by me this day on spindlers)