Also, the new PVP mode and more
MMO patches can often be bigger than full retail games. One of the reasons a subscription based game charges a fee is to pay for the maintenance -- the other is to pay for all the research and development that goes into new content.
In the case of Final Fantasy XIV, it's getting quite a bit of it in patch 2.3 -- a Ramuh boss fight (hard and extreme), three new dungeons, a new sidestory questline, a second portion of the raid Crystal Tower, a new 72-person PVP mode, monster hunts that take place on the world map, personal housing (in the form of a room for now) and a lot more.
Although it's a bit unconventional, expect a full review of patch 2.3 after release, and check out the trailer for a sneak peak above.