chorny,
True, but lets say he route in question is mainly used by business travellers. They want to be able to get from A-B in the morning, then home to A again that night, so will pay a premium for a more frequent service.
Basically, it all comes down to delta$, what's the most profitable combination of speed, size and frequency. It will be different for a particular route for any number of reasons, and change with different circumstances (like the way fuel prices are seeing once desirable 50 seat jets gatering sand in the desert), but there is never one easy answer.