Jet2 is NCL's largest airline in terms of aircraft based, routes etc... If they were to open up some more year round routes and more winter routes they would without a doubt be the largest airline. I think it's routes like BRS, BFS and AMS that keep EZY the largest (as far as I know) in terms of passenger numbers because they operate 2/3 daily flights every day all year round along with other summer/winter seasonal routes.
If we ever get the 8 based aircraft from Jet2 I really hope to see more city routes. Berlin, Budapest, Vienna, Verona, Reykjavik, Marrakech.
EZY could expand at NCL using non based aircraft, did they ever try Hamburg or Lisbon?