A concave mirror of small aperture forms a sharper image. Why?

The rays of light travelling parallel to the principal axis, after reflection from a concave mirror meet at a single point only, if the beam of light is narrow or if the mirror is of small aperture. In case, a wide beam of light falls on a concave mirror of large aperture, the rays after reflection from the mirror do not come to focus at a single point. Therefore, it follows that, if the aperture of the concave mirror is small, the image formed will be sharper.