They do not earn money from a full plane, but money from a plane two thirds full whose passengers are spending more money on priority boarding, luggage, credit card fees, travel insurance, on-board sales. And, first and foremost, they earn money from airports providing services for (almost) free and from tourist authorities paying for an airline service ("marketing support")
I do not know much about the spending habits of Scandinavian people in comparison to Germans, Britons or Eastern Europeans, but the issue is very interesting. Namely, where the most important sources of Ryanair's income come from. Indeed, the mentioned Tampere authorities probably do not pay any marketing support (let's stick to this example). But isn't it pretty lucrative to have those Finnish guys for a holiday (drinking vodka and having fun) in other parts of Europe - why nobody wanted to expand this "marketing support" towards them? And if this "support" is so essential for Ryanair, why on Earth are they speeding the development of the domestic routes in many countries nowadays? Will the Lille authorities pay for having the guests from Marseille or vice versa? Somehow I doubt it.