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.
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Emmanuel Puybaret, Sweet Home 3D creator
