Airlines get hit in a number of ways:
1. Costs for landing and take off - based (usually) on weight
2. Costs for parking - usually based on weight, but other factors may come in (wingspan etc)
3. Pax handling - imposed by the airport - sometimes on landed pax, sometimes on boarded pax - seriously Nazi airports impose both
4. APD - £5 to £40, on departing pax, based on class and destination
5. Security and anything else they can get away with.
Items 1-5 except 4 are at the discretion of the airport!
TA