why do things look smaller...

why do things look smaller when you get farther away from them?

The perceived size of a thing is proportional to the size of its image on the retina.  Each small spot on the retina samples light that comes from a roughly cone-shaped region of space.  Now, consider an object with an image perpendicular to the viewing axis that is enclosed by a circle.  If you change the distance from your eye(s) to the object, then the apex angle of the cone whose base is that circle and whose apex is the center of your eye lens decreases.  The size of your eye does not change, so the image of the object on the retina must shrink, and the object appears smaller.