Visual Weight
The degree to which an element in the frame attracts and holds attention, independent of its physical size.
In depth
Weight is increased by brightness, saturation, sharpness, contrast against surroundings, human features and isolation. A face carries enormous weight even when tiny; a large expanse of even sky carries almost none. Composing is largely the business of arranging these weights so the frame neither tips nor stalls.
Examples in practice
A red door occupying two per cent of the frame outweighs the entire grey wall around it.