The width is dynamically generated based on the widest image in each album. If you browse the images, you will find at least one that requires all that space.
By default GSP should be creating smaller compressed versions of your high-res originals. Have you changed the option that displays your original images rather than the compressed ones? Or maybe you adjusted the size of the compressed images in the Site admin area?
If you provide an URL to the album, I can take a closer look.
Roger Martin
Creator and Lead Developer of Gallery Server Pro