So these table surfaces are clearly very different sizes, right?
Wrong. Click on the picture to see a demonstration I’ve just prepared for you. If you think I’ve cheated, check it yourself with a ruler, or a piece of paper. What amazed me more when I was preparing the demo today is that the illusion isn’t established only by the table legs (which is the first thing that comes to mind): we still see them in different sizes when the legs disappear. The rotation is sufficient to deceive us, the legs just strengthen it.
By the way, the reason they don’t overlap exactly is that the corner angle is slightly different; the side lengths are really equal.
Lesson learned: use your guides and ruler in Photoshop!