Generally, the eyes of nocturnal animals —like small cats— have multifocal lenses that allows them to increase contrast and depth of field in low light conditions. A circular pupil, contracting to protect the eye against bright light, would interfere with this type of structure (the iris shades the peripheral zones of the lens) leading to the loss of well-focused images. The slit pupils, therefore, may have developed in association with multifocal optical systems because more effective.
Vice versa, big cats are generally diurnal predators and they have monofocal eyes like us. So, their pupils tend to be circular because they are an adequate adaptation to monofocal optical systems.