jabird,
As I said I don't know B737-800 performance but, off a restricted runway, the longer the route then the fewer passengers can be carried ... then this will be impeded further by such things as unexpected headwinds, bad weather at destination etc. ... By far easier to operate from a runway that has room to spare, keep things simple!
As for fuel burn it's circa 4% of the passenger/baggage weight that is burnt or not burnt per flight hour and as for MOL and his beancounters ... empty seats don't buy curled up sandwiches, lukewarm beer and/or coffee nor do empty seats pay £1 for the privilege of disposing of the consumed sandwiches, lukewarm beer and/or coffee, nor do empty seats pay rip off credit/debit card charges etc. etc. etc.