According to About.com, the idea that darker or deeper colors make a room look smaller is not always true. Rather, it is the contrast between walls and trim that makes a space seem smaller by calling attention to the edges and borders of a room.
According to About.com, when the walls are a dark color and the trim is white or a different lighter shade, the eye takes notice of that division. However, if the trim is painted a deeper color as well, the eye does not stop at the border but continues around the room. It is because of this that the space can feel more expansive.