The reason the moon looks bigger at the horizon than overhead is that the moon is bigger at the horizon than it is overhead. There are sound physical principles that explain why this happens. As you know, the moon is in orbit around the earth and if you recall your physics you will remember that objects in orbit maintain constant potential energy. You will also recall that potential energy in a gravitational field is a function of an object's mass and altitude relative to a reference plane. When the moon is on the horizon, lower to the ground, it has a certain, defined potential energy. As the moon rises, it must conserve energy and therefore must shed mass. If the moon did not shed mass, it could not rise or it would be in clear violation of well established physical laws. The moon continues to shed mass until it reaches its maximum altitude at its azimuth (named for the famed English mathematician/ automotive designer Sir Isaac Azimuth who invented the differential and who should not be confused with the other famed Azimuth, the sci-fi writer and spectular humorist{one who tells off-color jokes}). After passing through its azimuth, the moon begins to accumulate mass at just the right rate to maintain its orbit. Now you may well be asking yourself "what happens to the mass the moon has shed and how does it get it back?" The answer to the first part of this question was first answered in the last century by Einstein's famed equation which shows that moon mass is converted to energy and shed it the form of moonbeams. The second part of the question "how does the moon regain mass?" has been a mystery until very recently. I don't claim to have a grasp on the fine points of the theory but I must direct you to the recent works of XXX*.