Close your thumb to your eyes and align your eyes, thumb and moon. Then focus your eyes on your thumb and watch the moon behind your thumb. You will find that the moon becomes small! Remove your thumb and look directly at the moon with your eyes. The moon becomes big again. Why is that? My explanation is that when you focus on the thumb close to the eye, the focus of the eye is very short. If you look at the moon with a short focal length, it will appear smaller. When you move your thumb away and look directly at the moon, the focal length of your eyes becomes longer, and when you look at the moon with a long focal length, the moon becomes larger.