Remember that the closer your subject is to the lightsource, the softer the light will be.
- CJ
MK, I think the answer to your question is something along the lines of, the further a lightsource is from the model the smaller that source becomes and so creates sharper deeper shadow with more fall off.
The closer the lightsource, the bigger it is in relation to the subject and therfore throws a wider beam of light onto the model with less/softer shadow. ... at least thats my understanding of it, Im sure CJ will correct me if Im wrong.
Remember that the closer your subject is to the lightsource, the softer the light will be.
I think it is worth noting that this only applies when the "lightsource" is diffused in some way. Natural light through a window is very different from a naked flashgun head. To get softer lighting the lightsource needs to be bigger relative to the subject. Moving nearer the window will do this, moving closer to the flash head won't.
The quanity of light problem is actually tied up in the same laws of physics as the softness and is all to do with the angles involved,