Because the angle of which light hits reveals how much of the moon we see.

The moon orbits around earth because of gravitational pull, And the sun stays in place as earth orbits around it. So the sun is shining light in all directions. But as earth spins only a half the earth gets light at a time. That also apply for the moon the moon is rotating around earth only getting a certain amount of light at a time. sometime the moon gets no light and that happens when there is an eclipse. But we as humans on the earth can only see a portion of the moon at a time, that also affects what portion of the moon we see. Because only some of the moon is getting light so even tho at times the moon looks small we are only seeing the portion of the moon. sometimes that portion we see has little light on it or a lot of light on it.