The Moon doesn’t actually get smaller or larger during its phases. The changes we see in the Moon result from the fact that the Moon orbits Earth.
The Moon reflects the light the Sun shines on it. When the Moon passes between Earth and the Sun in its orbit, we don’t see the Moon at all because the sunlight is hitting the side facing away from us. This is the phase we call the New Moon.
As the Moon’s position changes in relation to Earth and the Sun, we see either less and less, or more and more, of the Moon’s surface with each passing night.
When we see the whole face of the Moon bathed in sunlight, it appears round and bright, a Full Moon. Each night, as the Moon orbits, we see less and less of this lighted portion.
As the Full Moon becomes a New Moon (or wanes), the phases are known as Old Gibbous, Last Quarter, and Old Crescent. As the New Moon becomes a Full Moon (or waxes), the phases are named New Crescent, First Quarter, and New Gibbous.
A person weighing 100 pounds (45 kilograms) on Earth would weigh one-sixth as much on the Moon—about 17 pounds (7.65 kilograms).