Even though imo there's been way too much discussion about the enterprise's new size already, my 2 cents..
Bigger isn't necessarily better. Modern aircraft carriers get bigger and bigger for a reason - they need the space to operate, maintain and store their attached aircraft, which tend to be quite big - a hornet is quite longer then a galileo shuttlecraft..
However the Enterprise was never a carrier, it was always more of a jack of all trades - sure it's got a few shuttles, but it's main weapons are phasers and photon torpedoes. The new one could be seen as analogous to a heavy cruiser.
Modern day's warships, apart from carriers, tend to be a lot smaller and far less armored then their ww2 counterparts, relying on advanced detection equipment and long range weaponry to deal with their targets instead. The heavily armored battleships are a thing of the past - even though they could have endured far more fire then an aegis cruiser, modern technology allows the new ship to deal with it's targets long before it comes into range of the enemy's guns.
My point is, if you can get something to be smaller, yet get it to do the same or better job, with less required crew aboard to boot, then you really want to do that - the result is a ship that is faster, more maneuverable, has a smaller target profile, puts less men at risk, and is probably cheaper to move and maintain.
I see the new constitution class as a more heavily armed variant of the old one, federation probably wanted ships with more teeth in this timeline - and considering the technology of that age, the only way they could accomplish that was with a much larger vessel. Doesn't mean the subsequent Galaxy and Sovereign classes in this timeline (if they'll even exist) will necessarily be larger then their original timeline counterparts.