In this video the two guys are at some illusion exhibit in New Zealand. Apparently this room in the exhibit is built to be an optical illusion where when a person walks from one end of the room to the other it looks like the person is turning into a giant. This is the same depth of field effect they use in movies to depict giant size characters. I only notice a little bit of the right side of the room being "lower" in the camera view. It's possible that the right side is the same height as the left side but the floor sure looks slanted. Anyway, the right side is much farther back (or so it seems) that the left side of the room. As the person walks from right to left I assume they get closer to the camera (and I assume the floor to ceiling height is also less).
Watch the full video of the giant in the room illusion below or click over and view it on the host site.