Why does the moon look bigger when it's rising than when it's high in the sky?

Optical illusion, there is no magnification due to atmospheric density or any thing else. If you took a photo of the rising moon just above the horizon and again 6 hours later at its zenith, then measured the diameter of the discs, you would find that they are identical.

I think it has to do with the angle of the horizon

Indeed, the moon is always roughly the same size in the sky (about half a degree in diameter), although it gets a tiny bit bigger when it’s closer to us in its orbit and a tiny bit smaller with it’s farthest away. But the difference isn’t measurable without instruments of some kind. The moon’s apparent increase in size near the horizon is basically an optical illusion—because you have a sense of the size of the building or the mountain on the horizon, you see the moon “next” to it, and the moon looks huge. Without that context—i.e., seeing the moon high in the sky, with nothing around it—it tends to look pretty small. Ryan Wyatt Rose Center for Earth & Space New York, New York

When you look at the moon, rays of moonlight converge and form an image about 0.15 mm wide in the back of your eye. High moons and low moons make the same sized spot. So why does your brain think one is bigger than the other? After all these years, scientists still aren't sure why. A similar illusion was discovered in 1913 by Mario Ponzo, who drew two identical bars across a pair of converging lines, like the railroad tracks pictured left. The upper yellow bar looks wider because it spans a greater apparent distance between the rails. This is the "Ponzo Illusion." Right: The Ponzo Illusion. Image credit: Dr. Tony Phillips. [More] Some researchers believe that the Moon Illusion is Ponzo's Illusion, with trees and houses playing the role of Ponzo's converging lines. Foreground objects trick your brain into thinking the moon is bigger than it really is. But there's a problem. Airline pilots flying at very high altitudes sometimes experience the Moon Illusion without any objects in the foreground. What tricks their eyes? Maybe it's the shape of the sky. Humans perceive the sky as a flattened dome, with the zenith nearby and the horizon far away. It makes sense: Birds flying overhead are closer than birds on the horizon. When the moon is near the horizon, your brain, trained by watching birds, miscalculates the moon's true distance and size. Above: The 'flattened sky' model for the Moon Illusion. [More] There are other explanations, too. It doesn't matter which is correct, though, if all you want to do is see a big beautiful moon. The best time to look is around moonrise, when the moon is peeking through trees and houses or over mountain ridges, doing its best to trick you. The table below (scroll down) lists moonrise times for selected US cities. A fun activity: Look at the moon directly and then through a narrow opening of some kind. For example, 'pinch' the moon between your thumb and forefinger or view it through a cardboard tube, which hides the foreground terrain. Can you make the optical illusion vanish? Stop that! You won't want to miss the Moon Illusion.

refraction

because of light. as we know light travels in a straight path. an observer when looking to an object, it's always with respect to the surroundings of an object. this will give an imaginary plane perpendicular to the observer's eyes giving the comparison of the object to it's surroundings. in this scenario the atmosphere is your imaginary plane. the atmosphere's shape is with respect to the shape of the planet which is oblate spheroid. you are at the arc of the sphere, once the light waves reach the atmosphere, this will cause an illusion to the size of the moon since your plane is not a straight plane but a curved plane. that's why itlooks much bigger when it's rising and basically when it's falling...

because were crazy moose-heads