Interestingly, WN's busiest hub (233 departures daily) is now MDW (DAL ranks only 8th). MDW is not a large airport, and cannot grow (surrounded by city). If the market demand says WN could fly 50% more willing customers out of MDW...but they can't use more planes/departures (gates already full).....
I didn't think the 767 could operate into MDW as the runways are too short?