I think the sollution would be quite easy to implement:
Make the limitation of planes based on the size of the airport (infrastructure), not the demand in the surroundings. You already have that for all airports, maybe just “translate” the size back to a number (i guess it is a “size_id” in the database already?) that can be used instead of the “bars”.
And:
MUC may have smaller demand levels than STR, but still the airport is not only built for local demand but for usage as a hub! The visualization “at it is” is fine now: you see how high the potential (!) demand is. MGL COULD serve a lot of people, but the infrastructure limits its operations.
Idea for additional realism:
Not only implement “slots” in terms of “runway operations”, but also “gates” in terms of parallel handling of aircrafts. You still can fly 4 planes per 5 mins into MGL, but MGL can only handle 6 planes in parallel (see the parking positions in google maps). More than 6 planes handled heavily increases turn around time! It is not easy to implement, it would act like “slots after slots”. Land at 8:00, but the next handling is available from 8:15 on. Or change from “runway slots” to “handling slots” that are used for the time of turnaround. The runway slots should fit to those from handling in real life? This would increase the use of terminals: you can increase the handling!
(edit: just found Redefinition of airport handling capacities)