system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the moon look so much smaller when you try to take a picture?
gpt
Not physics, psychology. And it's not just the Moon, lots of things that people take pictures of look smaller in the photo. Ever shoot pix at an airshow? Those planes that looked so big flying past are just specks.

This happens because when they were looking through the viewfinder, their brain "cropped" the image to the just part they were interested in. Amateur photographers don't look at the *entire* viewfinder, just the part that interests them. Result: pix where the subject looks a lot smaller than they remember, because they didn't notice how little of the frame it occupied when they shot it.

Next time you try this, take a moment and look at the entire viewfinder image, then estimate how much of that the Moon (or whatever) takes up. When you have the photo, you'll find that's exactly how big it turned out.

Professionals train themselves to look at the *entire* viewfinder scene by reflex.

Bona fides: former pro photographer, current serious amateur.