Leaving aside my issues with such a ginormous ground built Abramsverse-prise - Grrrr!
...it doesn't really make any sense to me, the U.S.S. Kelvin being that large.
There's the NX-01 through the Prime Universe NCC-1701, with that ship an unusual blip in constructions which by comparison, only got slightly larger inbetween.
It could be that those Kelvin-derived Newton, Armstrong, Mayflower and whatever else classes of ships seen in the last movie were all in service in the Prime timeline, concurrent to the TOS Enterprise, yet simply went unseen until now. They did it with the Remans, why not ships too?
The original series Enterprise could have been an equivalent ship to the 24th century's USS Voyager - a more advanced but smaller ship compared to the enourmous Enterprise-D's of the day.