Perspective.
You are asked, O Painter, why the figures you draw on a small scale according to the laws of perspective do not appear—notwithstanding the demonstration of distance—as large as real ones—their height being the same as in those painted on the wall.
And why [painted] objects seen at a small distance appear larger than the real ones?
The right position of the artist, when painting, and of the spectator (540-547)
540.