Thank you, Peter.
The output size (and the viewing size) does indeed affect the appearance of the lines. The larger the output size, the thinner and in this case harder the lines appear.
export.png
Here's an example. I used the same line style, the Straight 2B Pencil from above, and made a series of exports at different sizes. The smallest was 800 pixels wide which shows on the left. I then resized the larger images so they are all the same size. As you can see, by changing the export size you can vary the appearance of a single line style quite dramatically.
I often export lines at a larger size than the rest of the model so I can make the lines very thin. This also allows me to manipulate the image/textures and so on without affecting the lines.