As PDF documents are typically designed to be printed on paper, they often have large empty borders. xpdf is careful to include the entire page when selecting its default zoom level, so when I'm reading a PDF designed for printing quite a lot of screen space is wasted by default. Worse, if the page is taller than the screen, hitting the space bar will scroll down the page before moving to the next page; if all that's visible off the bottom of the screen is whitespace, then this doesn't achieve anything.
I would prefer it if xpdf calculated a bounding box around the actual contents of the page when selecting a zoom level and when scrolling, thus automatically cutting off the margins. For multipage documents, it could calculate the largest bounding box that includes all pages.