Light rays thru a small aperture can be close to perpendicular when they hit the imaging plane. So you can move the imagine plane forwards and backwards a bit, and the circle of confusion stays about the same size. That's a deep depth of field.
Light rays thru a wide aperture hit the imaging plane at a shallow angle. So if you move the imaging plane even a little bit the circle of confusion changes size dramatically.
https://en.wikipedia.org/wiki/File:Depth_of_field_illustration.svg
