Small planes means bigger frequencies. Your Landing fees and Other Taxes and Fees would increased dramatically if you place too much flights within one routes. Unless the routes are quite small, using ATR 72-500 wouldn't be much beneficial, also, there are BAE ATP, which have more range and faster speed. So A320 or B737 would do the job better on most routes.
Not really.... Landing fees and other taxes are much smaller for a small plane. The only same expense is flight crew and a bit more of ground crew, but they're relatively cheap.
Especially if you fly domestic routes where you can build unlimited terminals (and they count as evaluation and take only a fraction of expense as leased gates) with smaller planes, the profits should be great.
Not to mention that bigger frequency for a large plane takes more turn time and sometimes holds a big plane more on the ground than in the air on a short route... That means zero profit when it's held on the ground, but I guess a lot of people don't know much about turn time.
Which is all why regional planes make a lot of sense.
By the way, some other profitable Russian planes, Yak-42 shouldn't do much worse then a 733, and An-24 is also a simple prop not much different then e.g. F-27.