I feel compelled to point out yet again that the phasers, torpedoes, airlocks, and the hull plating is probably how the original 1701 might have ended up looking had TOS had a bigger budget. That and Gene told Matt Jeffries not to put a lot of detail on the hull.
As for the different proportions, going by the alternate timeline, it's reasonable to assume that the Constitution class was in the early planning stages when the Kelvin was attacked by Nero's ship. After that ship was destroyed, Starfleet probably overhauled the entire design. Notice that the saucer of the ship is much bigger than the engineering hull, thereby presenting a larger target. While that sounds stupid at first, think about it this way. The saucer does NOT contain a matter/antimatter reactor that if something goes wrong with it, ie. battle damage, the whole thing blows up. And by making the engineering hull smaller, it is a lot harder to hit and reduces the risk of a lucky shot taking out the warp core and destroying the ship. The engines and pylons moving forward also reduces the overall length and size of the ship, again making it a smaller target. It also is more balanced since a good portion of the ship is closer to the center of gravity. Also, the neck of the Enterprise is thicker so there is less opportunity for a torpedo barrage to forcibly separate the saucer from the engineering hull.
