Monocular Cue: Relative Size

Monocular cues are depth cues available to either eye alone.
One of many monocular cues is relative size. If two objects are assumed to be similar in size, the one that casts the smaller retinal image is perceived as farther away.

For example, in the picture below, we perceive the large black ball with an 8 as closer to us than the smaller orange ball which is farther away. However, we know that both balls are of the same size, yet since the ball that is farther away takes up less of our field of view, we can perceive the distance between both balls.