"My concern is that there is a limit to going on putting big aircraft like the 738 into small markets."
Conventional aviation logic would agree with you, but Ryanair logic works differently. The 737-800 is about as good as it gets in terms of crew: passenger ratios, and is similar to a 737-200 on fuel costs, but with 55% more seats. Therefore on thin routes which other carriers might not touch, Ryanair are able to make a profit using this larger aircraft.
I'm sure MOL's done the maths, and it would go along the lines of - extra rotation to Dublin at €50 per head or take people to Dinard for €30 per head, but 50% of pax take a hire car at €200 per booking, of which FR get 30% (50% x 30% x €200 = €30), so Dinard effectively brings in €60 per head.
I don't know if any such breakdowns are ever published on this, so my speculation may be a little out.
Sooner or later they are going to have to go into more big airports.
Why would they need to? Look at the places they currently serve, add in the hundres of airports they will be talking to at any one time, and consider all the combinations for dots which could be drawn up on the network. Larger airports = larger delays = higher expenses.
I can think of numerous routes where they could find rich picking grounds. LON-MAD is very vulnerable to competition just now as BA and IB try to grapple with their absurd pooling of services.
Don't discount FR from looking at Don Quixote, but Barajas would be too congested and high cost for them.
AGP would be an easy market for RYR from STN, DUB, ORK, Hahn, Lubeck, Torp and Stockholm.
Plenty of other less busy airports on the Spanish Costas, but AGP is already served from DUB.
ATH is crying out for more services.
SKG desperately needs low fares.
Too far?