Relative size is the depth cue that allows us to establish and estimate the distance of an object, by comparing the said object relative to other objects around. This depth cue is closely related to size constancy; we perceive an object as further when it casts a smaller retinal image compared to an object that casts a larger retinal image.
The picture above shows two balls of the same size; a yellow ball and the blue ball. As you can see, the image of the blue ball looks smaller as compared to the image of the yellow ball. Relative size enables us to know that the blue ball is at a further distance compared to the yellow ball.
The picture above illustrates to us about the concept of size constancy. In the first image, although we can see that the image of the man in blue is smaller than the image of the man in red, we do not perceive the man in blue to actually be smaller in size as compared to the man in red.
Activity: Picture an image of two cars of the same size driving on the highway; from where you are standing, you see that one of the cars appears smaller than the other; you may perceive that the car that appears smaller must be further away from you than the car that appears bigger. The difference in size of the image you see may enable you to judge the distance of the car from your standing position. If the image of the object grows larger, you know that the object is moving closer towards you and if the image of the object grows smaller, you know that the object is moving away from you.