Look at the monsters in the drawing below, which is the larger one?
For most people, it would seem the larger monster is in the back. In reality though, they are both the exact same size. It is the depth perception cues in the animation (see receding tunnel), which gives the 2D image a 3D effect. So although both monsters create the very same image size in our eyes, our brains take the depth cue factors into account. This results in the perception that the monster in the back is further away and that it appears that much larger!