The interior box has mirror both on the inside of the door and the back of the cabinet.  The double mirror on the door makes it rather heavy.  I do not have it installed yet and have some concern about its weight, but it has received no complaints that I can find online.  The interior box is not that much shorter or narrower than the mirror itself.  If you are in need of a sizable amount of extra storage tucked into your wall, this is certainly the mirror for you!