Why does my white tree frog have narrow eye pupils?

Generally, the pupil size depends on the lighting. The pupils tend to get bigger at night and in low light, and narrow in bright light. If you're wondering about the slit shape of the pupil, this is common in nocturnal animals (including cats, who are naturally somewhat nocturnal). In predators, like cats and snakes, the pupils are usually vertical, which helps with gauging depth and narrowing in on their prey. In prey, like some frogs and toads, they tend to be horizontal, which helps expand their field of vision and keep them aware of their surroundings.