Question

How is aperture of spherical mirror lesser than the radius of the mirror? ​

Answer 1

Spherical mirrors do not focus parallel light rays to a single focal point. The focus actually lies on a surface called a caustic. Wolfram have a nice demonstration of this here.

Light rays close to the centre of the mirror, i.e. the region where the mirror is only slightly angled away from the vertical, do come to a single focal point to a good approximation. This is known as the paraxial approximation. The point of using an aperture much smaller that the radius of the mirror is to restrict the incoming light to a region where rays do (almost) come to a single focal point.