Interestingly, it is because your brain is so used to dealing with perspective (the real sort of perspective, not this fantasy land crap the flat-earthers are talking about) that causes the Sun and Moon to seem bigger when close to the horizon when they actually are just exactly the same size. When the Sun or Moon is close to the horizon, your brain interprets the object as being approximately the same distance away as whatever is on the horizon. When they are high in the sky they seem further away, even though they aren't. Your brain compensates for the perceived difference in distance by making the lower Sun or Moon seem larger because closer things are supposed to look larger.
The effect goes away if all you do is take a photograph of the Sun or Moon. The Moon is the easier one to deal with since it hurts to look at the Sun and since the Sun and Moon are pretty much exactly the same size in the sky then it'll do nicely as a stand in for the Sun in this regard. If you go out and take a picture of the Moon you'll discover that it is actually quite a bit smaller in the sky than it seems, whether its on the horizon or not. An unmagnified photograph of the Moon (i.e. a photo taken with a lens in the 35-50mm range) can hardly be recognized as being a photo of the Moon at all. It's a tiny little bright spot in your otherwise dark photo.
Here's the best example i could find of the Moon photographed with a 50mm lens. A 50mm lens gives a slightly smaller field of view as the human eye and the photo has probably been cropped slightly and, as a result, has been slightly magnified and even still its a pretty tiny thing in the sky.
Click on the image to see full sized version...
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In short, your eyes aren't cameras. The fact that they are hooked to your brain means that the image isn't about simply reporting/recording the data.
Clete