You can control the line thickness with File>Preferences>General Preferences>Cursor>Circular Cursor Size, the + and - keys on the numeric keyboard, or the mouse wheel.
The lines you see are representations of three-dimensional cylinders and frusta (i.e., sections of a cone, like a cylinder, but tapered). When the 3-dimensional tracing is displayed on the flat screen, the result is a thick line if thickness is turned on.