why is the moon size decreases after the full moon ?
The moon has phases in which the moon takes different shapes and sizes.A full moon is just one phase . The moon decreases because it enters another phase which is smaller in size than a full moon. The “size” of the moon— the diameter of its disc, seen from earth— varies about 10%, since that’s how elliptical its orbit around the earth is. At its farthest, it’s about 10% farther away than it is at its nearest. Those farthest/nearest points in its orbit have no connection to its phases. Full moon can happen when the moon is close to the earth (as it just did), or at any other time.