Are you importing back into SolidWorks or another program? When I import into another program all lines come in very thin, when I import back into SolidWorks, sometimes they can look thick but when I zoom in, again they are very thin.
I don't know how you can change it from SW but on autocad you can modify the complete with display lines with tipe on command line: lwdisplay if is "on" turn it "ff"
If you're exporting an SW drawing, then the line widths are controlled by what's set in the layer dialog.
In DraftSight, check the line weight column in the layer dialog.
If Default is shown, go to Format>Line Weight and see what the default value I set at.
I am importing into Cadra, but did check to see how they import into eDrawings and get the same result. At first I thought maybe the issue was the Cadra software importing the file, but I have had customers that use AutoCad and Pro/E tell me that they have issues with the files as well. I do not know where to find the DraftSight that Kevin mentioned?
I have also found that my gtol.sym file is missing.