Depending on location/the set up/other factors birds may not only use the coop for roosting though. I've had to lock my flock inside twice due to flooding, since the coop is elevated, and during snowy weather some birds don't want to come out.Does it matter how many square feet the henhouse is, since they only roost there? As long as they have 12' per hen roost space, isn't that what's important?
For integration, having at least 4 sq ft per bird also helps provide extra buffer space during the days/weeks it takes to get the adults to learn to share the space with the chicks, and vice versa.
And overall, a coop that's smaller is also going to be harder to properly ventilate and may be harder to clean (depending on size of tools you're using) or layout (i.e. roosts, because birds need landing space in front of the roosts).