Modifier size vs. subject size

I need a primer on modifier size. Lets assume all else being equal, 200ws strobes, using a softbox or octobox, what is the advantage of say a 40" octobox vs. a 70" octobox?

larger light source = softer light at same distance from subject.

OTOH, if you back up that 70 inch softbox, so it's the same size *relative* to the 40" at the original distance you'll get less falloff on the subject front/back . . .

OK, so if you are shooting a family of four you aren't going to want the light super close because it will cause uneven light between the subjects. If you move it away, you lose apparent light source size. So the larger your light source is to start with, the better. Is that right?

You can't say that larger is better, because it all depends on the size of the subject and what quality of light you want.

But your example is accurate. So, it just depends on what you're going to be shooting most often.