The default line weight is controlled by the LWDEFAULT system variable. You can set it to whatever you want.
According to help,
The default lineweight can be set to any valid lineweight value in hundredths of millimeters, including: 0, 5, 9, 13, 15, 18, 20, 25, 30, 35, 40, 50, 53, 60, 70, 80, 90, 100, 106, 120, 140, 158, 200, and 211.
All values must be entered in hundredths of millimeters. (Multiply a value by 2540 to convert values from inches to hundredths of millimeters.)
Are you plotting a layout or from MS? My default is set to 25 and when I made a .ctb and set a color to print to .25mm the lines came out the same in that and monochrome when I plotted the layout.