Ah, the old "Square Foot Rule." SO often batted around, and just as often misunderstood.
You have heard the classic answer, so I wont repeat it. I will only say, at the risk of offending, that it is one of the most worn out fallacies in the chicken keeping world. Unless you have a lot of outside room for them to run and feed in (you did know they should be fed outside the coop, right?) so they will only roost and lay in the chicken house, these numbers are absolute minimum...and much closer to ridiculous in practice.
The oft quoted figures are for indoor, square footage on the floor of a coop, correct? Let's try it, shall we?
Go out to the garage and GET 16 floor tiles and lay them down on the floor of your kitchen in a square, 4 on a side. Or, if you have them already in place, good. THAT, my friends, is what you are allotting per adult bird, an active creature that handily fills 18-24 CUBIC INCHES of space.
Now find a five gallon bucket to represent the bird's volume of space and put it in the middle of the tiles. You have just simulated the reality of the thing. None of this changes for banties except the scale.
The square footage figures we are referring to here are offshoots of intensive management systems, the epitome of which are seen in factory rearing. It works in a tightly managed system, with genetically manipulated birds that only live for 8-10 weeks. You don't even want to ask what is alloted for a factory layer. But, in the small flock, under average hobbyist management (or mis-management), these are formulas for disappointment - or worse.
In the old days, 10-20 square feet in the coop was recommended, as overcrowding is one of the cardinal sins of keeping any livestock. Chickens are no different. Just for giggles, guess what they considered to be the maximum outdoor space allotment a century ago - back before anyone had heard of factory rearing?
500 birds/acre, or 87.12 square feet per bird.
This was considered the max under intense management and it was often rounded up to 100 sq.ft. to ensure a margin for error. Believe it or not, I don't make this stuff up!
Want your chicken keeping to be a pleasant experience? Wanna come close to guaranteeing success at it? Then reduce their numbers, give them as much room as you can and make it more than you think they need... then toss that 4 sq. foot rule out the window.