Not all Bmi regional flights operate to/from "regional" airports and just because they operate smaller aircraft does not necessarily mean they are quicker to turnround. It can take more staff to board an Embraer than an Airbus depending on the set-up of the stand! You also have the added job of removing and tagging the larger items of handbaggage which would normally be Ok. The fact that BD operate less flights then BA does not make a difference as it is a statistical average, the size of the operation makes no difference.