Mistaken mirror box

In the movie, the mirror is briefly shown to be angled, but it is angled the wrong way. It should have been angled with the near part at the bottom, and the far part at the top, with the safe beneath, so that it would reflect the ceiling, not the floor.

As they have angled the mirror the opposite way, the whole of the mirror is in FRONT of the safe - and therefore there's no illusion of the full size of the room. They could just as well have used a fake vertical wall. In the magic trick, there are four identical sides to the box - unlike in a building, where floors roofs and ceiling typically aren't identical.

That the floor looks like the walls is in itself a cop out crated so the trick would make sense, but they just didn't understand the point of the angled mirror when they put all of the mirror in front of the safe. To crate the illusion the mirror has to lean in above the safe.