Hmm. I thought the VFX guys just made the ship whatever size looked cool for the specific scene.
I had read somewhere that the ship was originally designed to be a little bigger than the TMP version in size, but was scaled up double when the '09 shuttle landing scene looked too small. So now we have a ship twice(ish) the size of the original but with the external details being too big for its size (two deck tall windows in some areas, and decks without any windows at all).
I tend to think of the ship in that smaller size, so my little toy TOS and REBOOT Enterprises can battle each other while being to equal scale.