A person getting into the room and walking along the wall opposite to the window appears to be growing larger/taller in size.
On the other hand, if a taller person stands near the door and a shorter one stands at the corner opposite to the window, the observer perceives the reversal in heights. The corner opposite to window is closer and shorter while the corner near the door is farther and longer. This causes the illusion.
This is a perception-related exhibit. As you observe a person inside the room walk along the back wall, his / her height apparently changes! This changing perception of height results from the way our brain uses the cues inside the room whose floor and the ceiling are geometrically different from what we normally find in rooms. We normally decide the heights of people in a normal room by observing the distance between the top of their heads and the ceiling. The lesser the distance, greater is the height of the person. This is valid for a normal room whose geometry is usually that of a cuboid. That is the floor and the ceiling are planes parallel to one another and the walls are generally rectangular. The room here is very different. The floor and the ceiling are tilted such that the distance between them is varying. It is larger near the end close to the door and becomes smaller as we move towards the end opposite to the window.
As a result of this the distance between a persons head and the ceiling decreases as he walks along the wall from the end close to the door towards the end opposite to the window. Hence the person observing this from the window perceives an increase in the height of the person walking inside the room.
Psychologists conduct experiments in such geometrically distorted rooms to understand the perception of size of an object by our brain.
Experiments such as these enable us to develop a deep sense of appreciation towards the functioning of the brain that we take for granted owing to its familiarity.