Physics of small aperture focusing

Essentially, the reason that you get a more focused image when you decrease the size of the aperture is because the photons of light that are coming into the camera are forced into a smaller space. When your aperture is wide open, the light is allowed to be scattered across a broader area, making the image blurry. By increasing the aperture and decreasing the size, the light is forced into a smaller area, thus creating a sharper image.