Proportion – Proportion is a correlation of size. It is the relative size of a figure on a given axis in relation to the other axis of the figure. For example, a rectangle that is 6” wide and 12” tall could be described generically as “taller than wide” or specifically as a “1 to 2 proportion.” Proportion can also describe one figure in relation to another figure: “twice as long” or “half as wide.”
In this example, the square to the left becomes the primary figure within the sequence because of motion. The motion is a change of proportion from a square to a vertical rectangle, back to a square, then to a horizontal rectangle. While there might be a tendency to describe this as a change of size, the area of the shape is unchanged. If the square was 1 square inch in volume, the change in proportion retains the 1 square inch of volume. It should be noted, however, that it IS a change in size as well because the shape becomes narrower or wider, but the more specific descriptor is Proportion.
If you stop the movie at the beginning or end, there is no clear primary figure due to all three squares being fundamentally identical (except for different positions).