Line thickness is more a combination of things rather than a single factor. These are, in no particular order, distance to subject, lower air pressure, thinner paint.
Generally, the closer you get to the surface, the narrower the spray pattern.
However, to spray at such a short distance (I'm talking 1/4" to 1/2"), you generally need to lower your air pressure to reduce "spidering" (or in the case of a line, "centipeding").
In lowering your air pressure, you need to thin your paint more so that it still flows smoothly through the brush and atomises correctly at lower pressure. Particularly with siphon feed brushes, if your paint is too thick, reduced air pressure means it won't "pick up" the paint. The thicker the paint is, the higher the air pressure required to atomise it into a fine pattern.
So really, you have to find a balance with the three factors above.
Edited to fix a number of typos