Proportion is defined as the size relationships within a composition. Proportion is super important and evident in every aspect of design. In photography, proportion is essential to keeping the photograph look real. In layout, proportion is used to create a sense of depth in a design, as well as create hierarchy.
As you know by now, I have created a design for each principle to give my viewers a real-life example. Below, the triangles of varying sizes, stacked atop one another, create a sense of depth. The larger ones appear to be in the foreground while the smaller ones fall into the background.