To clarify my previous post, part of the issue is the distance between the lens and your retina. In this case it's a two lens system, the glasses and your cornea, but a major contributor to magnification or de-magnification is the distance from the lenses to your retina and the distance from the lens to the observed object. I think maximum effect occurs when the lens is at the midpoint between object and observer.
Back to the person wearing the glasses, usually all imsages are de-magnified, since the image size on your retina is smaller than the actual size of an observed object. The variation is in the amount of demagnification.
It's related to the thin lens equation: