optics - Can someone please explain what happens on microscopic scale when an image becomes unfocused on a screen from a projector lens?

My questions is basically asking when you move a projector back farther from a screen the image tends to blur unless you focus it. Logically I would think that every point(ray of light) of the image would expand proportionally allowing just a larger clear image. Instead you get a blurred image that is larger and requires focusing. My question is what exactly is happening to the light when you create a larger distance between the projector and the screen. Why can't the image just get larger and stay clear without the need to focus. Is every light ray independent? So each ray of light expands and over laps the other when the image is not focused? If so what exactly is a single ray of light and how thick is it? Am I thinking too deep about this ? Why does such a simple concept seem impossible to explain?