I think as cruise ships get bigger, the "standard" of >2,000 being large is no longer as valid as it used to be. Especially as now that some of the mega ships are >5,000. According to a book I have, the Oasis of the Seas has 2,700 cabins so at 2/cabin it has a capacity of 5,400). It doesn't make sense to me classifying two ships as mega when one holds 3,000 and one holds 5,400. I would presume that a ship that holds 5,400 will have vastly more amenities and options than one that holds 3,000 and the process of embarkation/debarkation is a totally different experience so I have a hard time thinking of them in the same way.
I would personally classify Magic/Wonder as mid-size and the ones that hold less than 750 as niche or boutique. But every source categorizes differently. Even the book I used classifies large as >2,000 and has no mega category. But it also classifies Magic and Wonder as mid-sized based on 2 persons/cabin (1,750 capacity) as AnchorsAway mentions. I think it's best to look at how the source you're using classifies in order to understand: (a) how they calculate it; and (b) what it means/why they consider the size relevant (i.e., what does their large versus mid-size mean to them in terms of comfort, amenities, busy-ness, etc.).