The right and the left border of the navigation portlet are cut of with the font size configured in my firefox-profile. If I choose a bigger font size, the right column gets wider and navigation portlet looks ok.

My guess is that the right columns width is given in em (depending on the font size) and the images for the navigation portlet have a fixed width, so the right column has to be wide enough.

If the font-size is to big, the navigation portlet is becomming too small for the content type icons and the text.