You have been talking about sun as a source of light for this discussion. That's true SUN is far far far away making the little distance from subject to camera irrelavant. How do you explain, the same rule holds true when the light source is a studio flash/light? You, too, can take incident reading at the subject. Now, the light source, and point source at that, is only 10 feet away. You, the camera man/woman can be at 6 feet, 10 feet, or 20 feet, and incident reading is still valid.
The only rationale I can come up with is that the point source light falls on the subject. In this journey of the light, it spreads as it travels. Once the subject is illuminated, the reflected light doesn't spread. I don't know why and can't explain why.
Image size? OK, but the size of subject being 10% of the total frame or 100%, say just the cheek of the person or the whole face, at either magnifications, the spot in question, (let's say the cheek) is exposed identically. Does the size really matter??
Not arguing or disagreeing... I'm really confused now.