Relative size reveals depth through what the mind translates as closer and further from a viewer. If an image contains two automobiles of the same model and one is larger than the other, the larger automobile will be perceived as closer to the viewer. The is related to how linear perspective works, but relative size does not require the other aspects of linear perspective such as a vanishing point. Relative Size Relationship is a depth cue.

The figure to the left is perceived as closer than the figure to the right.

(relative size of many figures… the larger figures are closer to the viewer)
Georges Seurat. A Sunday Afternoon on the Island of La Grande Jatte. 1884-86.