As shown in the figure, the line A on the left represents the retinal plane. Line B represents the convex lens plane and M represents the moon.
When approaching the ground, due to the influence of the objects on the ground, the eyes will feel the moon is far away, so the eyes will use a longer focal length to image the moon (such as oa for its focal length). At the zenith, because there are no other objects, the eyes will feel the moon is closer, so the eyes will use a shorter focal length for the moon imaging (as shown in Figure sa for its focal length). The result is that the imaging ad of the former is larger than the imaging ab of the latter. So the observer will feel that the moon near the ground looks larger than the moon on the zenith.
So why is the moon on the ground as big as the moon on the zenith when photographed with a camera? Because the camera uses the same focal length to shoot the moon.
Is there any evidence to support my explanation?
Please put your thumb close to your eyes, and let your eyes, thumbs, and the moon align roughly at three points. Then focus your eyes on your thumb and watch the moon in the sky behind your thumb. Mother, you'll find that the moon is terribly small! Why? Because when you focus on your thumb, the focus of your eyes is very short. According to the theory introduced earlier, if you look at the moon with a short focal length, it will appear smaller. So conversely, when the focal length is longer, the moon looks larger.
This thumb experiment can be done by anyone, and it also confirms our theory.