The moon looks bigger when we
see it behind tall trees or buildings and are able to make
a size comparison. Our eyes are unable to adjust for the
true distance of the moon from the trees and buildings,
and we make our comparison as if the moon were merely a
little distance behind the trees.
After the moon is up in the sky, we however have no object
with which to compare its size. So it appears smaller.
The size of any object depends upon the angle they subtend
at our eyes. The moon in both the cases subtends equal angle
at our eyes. Its only that we see the size of objects relative
to some other object.
If you are still unclear about the concepts, please refer
the animation below.