And these are ships that are designed to do nothing whatsoever except cruise around and wait for orders to blow something up.
While they don't do the job of cruisers, they actually DO perform a variety of missions, particularly for sonar analysis, underwater research, search and rescue, communications coordination, etc. Their
primary role, of course, is sub-surface warfare, but the Navy has 'em doing a lot worse than that.
Again, we're only talking 400 people here. They do not need an internal area the size of a small city (of which the NuEnterprise is) in order to function. The Enterprise-D was rediculous in size for its crew of 1000, and wasn't quite as big. (To say nothing of the Lexington and other ships which were shown to be several times BIGGER again).
You want it to be bigger for no other reason than to justify the new movie, despite
the very fact that Abrams very pointedly considered such questions and discussions beneath him and even fired some technical advisors for doing their job.
Seriously, do some research on what
current ships really can do, with their crews, with
today's technology, at their current sizes. You'll easily discover that the TOS Enterprise was already pretty substantial a ship (remember, it was a bunch of WWII vets that designed her, and
served on such vessels, and brought that knowledge to the design of the ship), much less what theororhetical tech can do 15 generations from now.