Why the minimum coop size? ?

Where did it come from? How was it decided? I've got 2 bantums, and their coop is about double the minimum, so don't worry, I'm not trying to stuff them in a smaller space, I am just curious as to how the number came about. I've read lots of websites that state the minimum square footage, but none that really explain why, other than that's the smallest space they'd be comfortable. Thanks for any answer you may have!