Cyrano, from your graph if you take the continental figures the difference between 700 and 800 is quite small. This is due to the fact that the 800 has a very efficient cruise wing among other things. Now if you couple this to the low cost model which says that 20% of the passengers pay for 80% of the flight this difference is quickly eroded. Sure I accept that on an individual flight where pax numbers are low the smaller aircraft will be slightly cheaper. One less cabin crew member for one but the airline varies its schedule maximise loads and if it stopped getting that last 20% or part there of it would have to make more changes than just buying smaller aircraft.
Another factor that this doesn’t account for is that Ryanair by committing to a single type has managed to reduce both the purchase cost and the maintenance cost of the fleet. This means that each aircraft doesn’t need to produce such high revenue in order to be in profit. If your graph showed £1 profit per mile per aircraft I think it might tell a different story. Nice graph though.