Proportion has two meanings. The first is the relationship of one part of an object to another part of the object. A caricature is a good example of this. Note that the head is larger than the body, the front part of the head is larger than the back, the legs are longer, etc.
The second application of proportion is the relationship of an object to its surroundings.
This work by Marc Chagall shows several figures of varying sizes presented together.
This landscape by Carl Triebel emphasizes its vastness by putting in a human figure and animals to give a sense of that expanse.
Another very significant aspect of proportion is its function in linear perspective to make things that appear farther away smaller than those that seem to be closer, as seen in this work by Raphael - compare the human figures.
Proportion within arts with a serial component refers to the temporal lengths of the sections within the work, or in some cases, the relationship between a smaller group (or single performer) and the larger ensemble.
This relationship is distinct from that of scale which has a different meaning.