How to expand your inbox window

Is there anyway to expand (horizontally) my outlook inbox window? When I open an email, the message is "squeezed" in the middle (see image below). There is column to the right with nothing in it. Why can't you expand the inbox window to use this space? Right now it is painfully slow reading large emails  since you have keep dragging the position bar below back and forth while scrolling.

For what it's worth Yahoo mail has a similar page layout, but you can click a small arrow and make this right column disappear.

Any help would be appreciated.