Lol, I never have experienced that in my games, the margins on small planes are far better, compared in ROI as medium and large ones.
The amount of staff and infra costs you need to turn around a fleet of medium and large planes is enormous.
See here, 33% profit on turnover. I dont have that with medium and large planes.
Realize, this is an expert game, with broken event cooldown timers on the Oil Crises, Economy Crashes, Pandemic and Vulcanos. Which are running on my field non stop. Also, I disabled night fligths to cope with the bouncing flights of Volcanos. So, this is just daytime production.
I get 3200 income per flight atm on all my small landings, from all contracts. While I dont even turn around 100% of offered flights. So, I could extend my small stands if I wanted too.