I think there are only so many routes that can justify the size of aircraft used, especially by Ryanair the 800 with 189 seats. In the days of low 'ish' oil prices and the deal Ryanair done with Boeing this meant you could give away some of those seats. However with soaring oil prices this cannot continue!
I know Ryanair is finding routes now which had no problems on the 200/300 but cannot sustain the 800!
Surely as more and more aircraft get delivered and Europe gets more and more saturated with low cost routes the size of the aircraft will be important. I think Jet Blue have done the right thing and moved down a size. Far better to start a route with the smaller aircraft, then if the passengers appear move up to the larger aircraft.
Plus you will never have low cost going to airports who will not ‘do the deal’!