Service can be superb or egregious on any size ship, so let's leave that aside.
The remaining tradeoff is ports-of-call opportunity (smaller ships go more places) vs economies of scale (bigger ships cost less per Gross Ton to operate). O has chosen to use part of those economies to give pax "elbow room". We don't know about food venues yet, but I suspect the new ships will also have more venues like AquaMar and Bakery -- they'll have the room.
Frankly, there are not that many places that a 67,000GT ship can get into that an 86,000GT ship cannot, so there's not much opportunity lost there.
The new ships will sail with 61.4 GT/pax, noticeably less crowded than the 'O'- and 'A'-class's 55.8, and spacious compared to the 'R'-class's 50. For comparison, Oasis of the Seas offers but 40.5.