CrystalMaker's File > Export > (Pixel) Graphics command gives you WYSIWYG output, where the line widths are based on the current screen size, scaled by the output resolution. You can therefore make lines thicker by increasing the output resolution for a given plot size.
For example:
1000 × 1000 pixels @ 144 dpi (Thick lines)
1000 × 1000 pixels @ 72 dpi (Thin lines)
Note that images 1 and 2 above have the same number of pixels (1000 × 1000); it is only the "resolution" that differs - this is simply a value that is stored in the output file and which is interpreted by CrystalMaker in scaling the line widths. If the two images are displayed at the same screen size, then image 1 will have lines which are twice as thick as image 2.