The moon appears to change size depending on its location in the sky because of the phenomenon called the moon illusion. The apparent change in size occurs because of how the viewer's brain interprets the visual data it receives.
One theory suggests that when the moon is near the horizon, the viewer's brain interprets it as being farther away, therefore seeming larger than when it is directly overhead. Another theory argues that the relative size of nearby objects influences how big the moon appears. When the moon is near the horizon, the fine detail of terrestrial objects makes it seem larger. When the moon is directly overhead, there are no nearby objects for comparison, making it seem smaller in the empty night sky.