No it is not, you can prove by taking photographs of the moon at different elevations from horizon to the zenith and comparing them.
2. Is it an Illusion?
Yes it is an illusion, illusion of shape. At the horizon due to other close by objects the brain compares it and makes an illusion that the moon is big, but there is no object to compare in the sky and hence brain processes the moon as it is without any illusion. Like many of you might have seen many optical illusion pictures where they give two different shapes and one appears to be smaller but infact both are same if seen separately.. example as blow
The two circles are the same angular diameter. Yet many people judge the right one to be smaller. The two converging straight lines nearby influence our judgment. If the lines are replaced with solid black areas, the illusion remains.
The two figures are exactly the same size, yet many people judge the upper one to be smaller. But not all persons do. This illusion illustrates how our judgment of shape and size of an object can be influenced by the shapes and sizes of other objects nearby in the field of view.