Short answer - search for help on Pen Table and Drawing Line Weights
Longer answer - PTC hasn't been very good about line thickness for drawings. It is primitive, but adequate. Not as good as any other software I've used.
The main control is by line color mapping to pen number using a pen table. Each pen number can have a width associated with it. Geometry lines are assigned a color and hidden lines get a color; there are others. Part outline isn't an option. You can manually set line widths, but then you can't use a pen table, which would over-ride the widths you set.