Landing charges levied on each landing usually have a significant weight component (amongst other variable components like emissions, noise, handling and passenger charges) but the relationship usually isn't linear with passenger capacity. Landing slots required at busier airports to have the right to land at a certain time each week are generally traded between airlines with the slot coordinator's agreement with the value of the slot based mainly on the commercial attractiveness of the time slot.

Absolutely, good point. I don't know why I latched onto charges vs slots.