EZY can and are expanding their fleet...?
As per Wiki, they currently have over 180 aircraft on order. Some of these will be replacements, some will be expansion.
The simple fact of the matter is that if they could make the massive profits which some on here seem to think they can, then they could expand at NCL but they are choosing not to. These are facts - they aren't capacity restricted; so far, this year alone they have had 9 deliveries.
Edit: Just seen fa2fi's post which is very similar, apologies for my slight repetition. I do remember reading something about them having very flexible options depending on economic increase/decline if anyone has the link?