There is an old joke that goes, "that room is so small, you have to step outside to change your mind." If this applies to your bathroom, you have two options. One is to knock out walls and make it bigger; the other is to use your creativity and some visual tricks to make it look and feel larger.
Dark spaces always seem smaller than they are. If your tiny bathroom has a tiny window and tiny lights, it will feel even smaller than it is. The first thing to do is to brighten it up. If you can expand the size of the window to let in more daylight, great. Use frosted or brick glass to let in the light without exposing yourself to the neighbors. If you can't enlarge the window, consider a skylight or a light tube (Solatube is one manufacturer) to bring in more daylight. For night-time light, replace your old fixtures with bright, energy-saving light bars (the new LED lights are vibrant, consume little energy, and last a long time).
Casinos, ballrooms, and other commercial spaces use lots of mirrors to give the illusion of infinite space. This can also work for your small bathroom. Mirror squares are available at most hardware stores and can be used to create a "mirror wall", to make the room appear twice as large.
Other tricks include making use of normally unused space. Use a corner cabinet to give yourself more storage room without taking up valuable floor space. Use light, brightly colored paint or wallpaper. When you are finished, you may think you had knocked out that wall after all.