Proportion is the principle of design that deals with size relationships. The proportion of fruits to grains, for example, is a fraction of three to one. For every three servings of fruit, it’s recommended that you eat one of grains. Eating balanced is a good proportional tool for someone who is unfamiliar with it. In design, however, it means that all the furniture is to the same scale.
If you have a couch that measures seventy feet long when fully put together, it makes sense to have a bit of a blockier end table, or to have a large one with transparent parts, to create a sense of balance. The idea of keeping everything roughly the same size is a monotonous one, however. So designers use it to help create focal points by varying scales on the accessories or the patterns of the fabrics in the room.