For instance, size alone gives a certain indication of an object's importance. It indicates power, dominance. Or the repetition of elements. A repetition of the same thing over and over and over again can lead to certain sorts of meaning. A circle can represent continuity or wholeness.
Symbols work on different levels.
Just as they work in literature or art, they work in architecture. The Capitol's symbolism is not just in its design or physical details, but also in its openness. Fittingly for the emblem of American democracy, the grounds are unfenced and the building open to anyone who wishes to visit. Free access is a symbol, too. As a result, there is a constant underlying terrorist threat.