On loan from the library, I'm reading The Joy of Less by Francine Jay. It's a refreshing read, and chapter 7, Embrace Space, applied as much to my photography as to my home.
The space outlined in the viewfinder is like a container, holding it's greatest potential when it's empty. Space and the things we bring into the frame of a picture are mutually exclusive. For each new thing we bring into the scene, a little bit of space disappears. It is not necessary to fill all the space in the viewfinder.
Space is of equal value to things, or greater, depending on your perspective. –Francine May