The Bigger They Are One reason for the cat's excellent night vision is that its eyes have very large pupils.
Large pupils let more light into the retina.
The bigger the pupils are, the larger the lenses must be. Relative to their size, cats have larger lenses than we do. And cats that are more nocturnal, such as the lynx, have relatively larger lenses than more diurnal species, such as the puma. The pupil acts as a window into the eye, opening and shutting to control the amount of light that enters. In dim light, the pupil opens in order to let in more light.