You need to control the thickness of your lines inside Archicad.
The attributes manager is the best place to do it. You can set the lines to pt. (points) or mm (milimeters).
Make a simple drawing with your lines and plot the drawing.
Make any adjustment of the thickness and plot again until you are satisfied.
These should avoid your "fat" lines.
For hairlines inside Plotmaker, the only way I know is to go to OPTIONS, ALL PENS AND COLORS and tediously, change the thickness of each line used in the drawing.
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