The answer to this question is hidden inside the so-called "relative size" theory. The essence of this theory is that we perceive the size of objects not only due to their retinal size. The perceived size also depends on our perception of the other objects located in the surroundings. The zenith moon is surrounded only by empty sky, which makes it appear smaller to our eyes. When the Moon is rising, it is, on the contrary, surrounded by the objects near the horizon, and they exhibit a fine detail that makes our natural satellite look much larger. And that's it!

More Info: en.wikipedia.org