You're the first to report this feeling but in a way, I'm not so astonished by your comment. It's probably due to the fact that moving the mouse requires much less effort than moving one's body. [;)]
Human field of view is something not so easy to simulate too.
Nevertheless, try to increase the visitor field of view in 3D view modification dialog. A larger field of view might give you the impression you'll have more space.

By the way, if you still have some doubts, take pictures with a real camera and compare them with the 3D view computed by Sweet Home 3D. For your information, a 63° field of view matches a 28 mm wide angle lens of a 24x36 camera.