Given that the NuTrek films begin at a point between our only real points of reference (ENT where the ship had a crew in the 80s and "The Cage" where there was only just over 200 onboard) a ship such as the Kelvin having 800 survivors (because lets face it with the damage she takes there would have been losses) seems very out of place, especially as she seems to be acting as a survey ship--given that all other surveyors we've witnessed have all been relatively smaller than most with crew complements to match, add to that the fact that the Constitution-Class is deemed a heavy cruiser and on the forefront of Starfleet operations and exploration then would most likely be kept fully staffed as much as possible. This is for a ship that is meant to be within the Prime Universe, which makes it stand out compared to all other evidence we have available to us.
Whilst some of the changes in design aesthetics could be explained away (i.e. one of Starfleet's most prominent starship designers of the 2240s/50s was an engineer on the Kelvin who was killed during the battle, therefore someone else was given the assignment), that doesn't provide much of an explanation why the fleet suddenly feels the need to double the size of their ships, as it gives them no real advantage or improvement over what we've seen in the Prime Universe (whilst it may allow for more personnel and equipment, if anything making ships larger targets would be a noticeable tactical disadvantage--not exactly a logical compromise).
As from a production standpoint, I can only assume that they wanted to give the ships a bit more gravitas and make them more substantial, but in space with no point of reference then there isn't much point, the Enterprise could be 500 or 5000 meters long and it wouldn't be something that most would be able to discern watching the film. Since the Narada is coming from the future (from a time where we know the major powers are building bigger ships, and as a mining vessel she would be a considerably larger ship than most, given the need to processing and storage bays) then keeping the ships of the 23rd century at their original sizes would've worked just as effectively, showing them to be the underdog in the fight, which would mean having to rely on the guile and quick-thinking tactics Kirk really is famous for (so tension and drama can still be built up on whether or not the ship is going to make it through in one piece). Other than updating designs (either for modern cinema goers or if they had to due to not having the full rights for design elements) there is no real reason why the ships size had to be tampered with, at least none that are apparent.
