This illusion is called the Ames room after its creator, Adelbert Ames, Jr.
It is a very clever illusion. You think you are looking at a normal room.
But instead of a normal rectangular room, the room is made up of trapezoids
which, among other changes, makes the person on the left a lot farther away from you.
Since one figure is farther away than it appears, it is smaller on our retina.
We do not activate our depth perception properly and so see the person as smaller than he is.
This is the opposite of the Müller-Lyer illusion.
It is a similar illusion to that used to create the different sizes of Hobbits in The Lord of the Rings movies.
When Frodo rides in the cart with Gandalf at the beginning of The Fellowship of the Ring movie
Elijah Wood is actually sitting several feet behind Ian McKellen.
But the angle of the camera does not let you see the difference making Elijah Wood look hobbit sized.