Another factor is aircraft size. Several times when I flew out of BWI, instead of my crowded WN gate for a 737 (or two, if there were delays on one flight), I would wait across the aisle at a gate with just as much seating that had only a Comair regional jet to CVG.

The bigger difference vs. legacy airlines seems to be at outstations. I used to take a flight that departed CLE around 7-8 p.m. (BWI and MDW flights both left around the same time, so full waiting areas), so I would wait at the nearby United gates, which were done for the day, with signs announcing that the next flight to depart would be at 6 a.m. the next day.