RIS,
I think this has headed for a thread drift already, correct, in theory, that more seats per aircraft then the cheaper operating costs per seat but then one needs to fill those seats to justify the additional operating costs.
Ryanair didn't make mega profits operating the likes of BAC1-11's and B737-200's but since they went for an entire fleet of stretched B737's it would seems they've gone from strength to strength.
Easyjet, from the outset, have gone for the mid sized B737/Airbus, it is their business model and it seems to work for them.
On the other hand there was Buzz operating BAe146's, they didn't last too long, BMIBaby operating, I recall BMI cast offs, even B737-500's ... The rest is history.
Down where I am there is Cebu Air (Cebu Pacific Air) operating, short/medium haul, a mixed fleet of A319's, A320's, some A321's on order, and ATR72's ... A mixed fleet and it works for them, dependent upon the often restrictive lengths of runways in these parts they can chop and change aircraft types to accommodate more or less passengers on any given day.