The Kelvin was 1500 feet long, had a crew of around 800 and very industrial equipment. In the prime reality, Starfleet developed a 900 feet long Constitution class ship with lots of Jeffries tubes and automated equipment, bringing crew size to 430. In the alternate reality, the ships were not streamlined and grew to sizes of 2380 feet long, requiring crews of 1100 to maintain multiple warp cores, large bridges and industrial equipment.
That's my theory. In any case, whether you believe the new Enterprise is 1200 feet long or 2380 feet long, the Kelvin is 1500 feet long and is larger than the old Enterprise. Notice too how similar to 2230s Kelvin/Kobayashi Maru designs the new Enterprise is: large bridges with viewscreen windows.