You have to think of it this way -- when you shine a light through a piece of diffusion, the diffusion itself becomes the source of light on the subject.
And basically the softness of that light depends on its SIZE relative to the distance to the subject. This assumes that the light fills the diffusion surface evenly, as opposed to having a very hot spot in the center.
A good example of a very large soft source that creates very soft shadows is an overcast sky.
So a piece of diffusion clipped to some barndoors is maybe a 1'x1' square. This is less large than a diffusion frame that is 4'x4', and if both were the same distance to the subject, then the 4'x4' frame would create a softer shadow than the 1'x1' diffusion piece.
But as I said, it's the size relative to the distance to the subject. In other words, if you are the subject and you look over at a 4'x4' frame of diffusion to one side of you, and then look the other way and see a 20'x20' frame of diffusion but it is much farther back, then it's possible that the two frames of diffusion would appear to be the same size from your perspective, and thus produce a shadow of similar softness.
However, the difference between the two would be fall-off. If you are very close to a diffusion frame, which is now more or less the source of light, and you lean towards or away from the light, your brightness may change visibly. However, if that diffusion were much larger but farther away, and thus similar in softness, and you leaned closer and farther from that far-away diffusion frame, then there would be very little change in brightness.
