Have you ever wondered, after you took a photograph of the full moon that the Moon looks much smaller on the picture than as it appears to you in real? This effect occurs particularly when the Moon is above the horizon. This is a so-called Moon illusion, which is an optical illusion, meaning it can only be seen this way through our perception and is therefore not depicted on the photograph.
So what actually happens with this Moon illusion?
We see the Moon (or the Sun as well) above the horizon and in the surrounding there are usually a variety of other objects, like mountains, trees or buildings. These items give us information about the distance. This is called »depth information«. The Moon appears for us to be far away, because we can perceive the distance of objects around the Moon. When we look at the Moon high up in the sky, this information is missing and we estimate the distance to be less.
Now imagine, you put two balls in front of you, with different distance to you. The ball in the background will look smaller. If this further away positioned ball would look like the same size as the other, you would automatically presume, it has to be bigger than the ball closer to you. It is precisely this phenomenon that occurs with the Moon illusion: we see the Moon above the horizon and presume it is further away than when the Moon is high up in the sky and our brain tells us it must be bigger. This is why we perceive the Moon close to the horizon, to be bigger.
By the way, you can also »switch off« the Moon illusion by covering all objects around the Moon with your hands. Also a view from a headstand or through your spread legs to the back, let the Moon appear in its correct size.
The entire subject is quite complicated and still puzzles scientists. And as so often: it is not the Moon that tricks us, but we do it ourselves.