Fallacy.....with the proper ventilation, birds cannot 'keep a smaller coop warm' and in fact you will have problems with air quality....more room is always better.
I think someone just took the 1-2sqft for production facilities and doubled it.
IMO the 4/10 'rule' is a bare minimum, especially where I live and the fact that my birds are confined to coop and run.
Much depends on climate and management technique.
The more space and ventilation, the...