Actually the size of the Moon doesn't change much, but the distance between the Moon and Earth changes by as much as 75 meters per second due to factors such as non-circular orbit and influences from other celestial bodies. That's gravity for you. When a full moon approximately coincides with the farthest distance that the Moon reaches from Earth in its elliptic orbit, the lunar disk as seen from Earth will appear slightly smaller than usual. The technical term for this phenomenon is apogee syzygy, full Moon at apogee, or micromoon. The opposite terms are perigee syzygy, full Moon at perigee, or supermoon. Furthermore the Moon is spiraling away from Earth by an average of 3.8 cm per year. I'm basically quoting from Wikipedia here. The word syzygy intrigues me.