There are only two ways to truly make your bathroom bigger: expand to the exterior or push into adjoining interior rooms. The first option is very expensive. The second option is less expensive and more DIY-friendly, but you lose all or part of your home office, baby's room, or spare bedroom.
The third option is to work within your existing bathroom "envelope."
You can employ visual tricks to make the small bathroom look bigger (lighter-colored paint, different tile, tearing down shower walls, etc.).
Plus, you can shift things around within the bathroom to banish non-functioning items and increase usable space (namely, getting rid of hulking vanity cabinets).
Image: From About.com Video "Paint Colors for a Comforting Bathroom Style"; (c) About.com