For heavy breeds, the recommended 3-4 sq. ft. is indoors, with 10 sq. ft. in the run. If they are confined indoors all the time, it's 10 sq. ft. indoors. For a banty, I think it's 2 sq. ft. indoors and 5 sq. ft. in the run.
One of the things that can make a difference in coop size, is what the climate is like in your area. Whether you get weather that is really cold, get deep snow or that's very rainy, that drives your birds to spend more time indoors or whether you live in a place where the weather is milder. If you have a covered or partially covered run, that can extend the time your birds are outside in wet weather and also allow you to have their food, as well as their water outside.
The psychological/social aspect of chickens is a big considerations in space requirements as it effects aggression. Some breeds of chicken are more passive and some are more high strung. You don't want to get into problems with chickens pecking each other to death, due to over crowding. It's really hard to stop, once it gets started. Whenever you mix very passive breeds with more aggressive breeds, I think you also increase the risk of very severe pecking.
Another big consideration in space requirements, is, well, poop.
At 5 sq. ft. per chicken, you don't have a surface layer of crusted chicken poop that you have to either breakup and stir in or remove. At 4 sq. ft, you have some of that crusting and at 3 sq. ft. you have more. How much time do you want to spend cleaning your coop? Do you want to do it daily, weekly or once or twice a year? Do you want to manage a more crowded coop by using dropping boards under the roosts, that you have to scrape off? Do you want to use a sand pit under there, that you scoop every day? Do you want to just toss a little litter on occasionally, let the chickens scratch it up as it composts and shovel the deep litter out once a year?
The following is just my opinion, not directed at a particular person, just general feelings for anyone that is new and struggling to make these types of decisions. There are a lot of different ways to house chickens that work great and you really just have to pick out the one that will work for you. Most people starting out want more chickens than they need and want to build a smaller coop than they need. That is apparently just a quirk of being human. I don't think I've ever heard of a person that went wrong, choosing fewer chickens and a bigger coop. It's important to have what you want, but it's also important to have this be an an enjoyable experience and not turn sour, because of problems due to crowding.
The good news is, even if you make some choices that don't turn out well, you can always make changes as you go.