I like the Small/Med/Large hub idea...
As a small/tiny station guy myself, there is no need to account for much of the back office/ line support at a small station.
At a non-primary station of less than 30-40 flights, you don't need an HR office, for instance, especially as the computer age arrives. One of the Sups authorizes the payroll, and sends it to the accountant in India with the click of a button, where it is compiled with all the other stations, and sent along by another click to the domestic payroll company, that prints the checks, along with a file sent to HQ telling them how much they need to put in the payroll account, and E-deposit.
Line maintenance will almost always be contract, with no stores, or support for the Maint. Dept needed. Cargo is handled by a single contractor or two for all carriers. You don't need an Environmental Affairs officer, or a Training mngr, caterers, etc. All of these are function that can be carried out via HQ, or by contract, and that is where economies of scale kick in.
Even at a "medium" station, a lot of the backoffice/backramp stuff is now directed to HQ, rather than done locally.
Related to this is the gates idea some have floated. IMO, this has been largely solved, in the modern era, by flexible gating. You can pay-per-use now in many remodeled/new airport terminals. At least in North America, you are limited more by ramp space, than gate availability. And even that can be solved by remote stands.