The first time I ever saw the moon at the horizon, I was floored. It appeared so HUGE! But appearances can be deceiving for we see the moon at the horizon the same size [in fact, a bit smaller] as when it is high in the sky. The effect is called the Moon Illusion and, as photographers, we can take advantage of this illusion for some fascinating photos.
Though there are different explanations why the moon appears so huge at the horizon (and they are hotly disputed in scientific circles), the best-known explanation is the “relative size” theory, illustrated in this video by the classic Ebbinghaus illusion.