They re-imagined the Enterprise, just like they re-imagined the Klingons.
Well, not really.
After all, they took care to show the old
ship as well, in that Rec Deck display of images. TOS wasn't supposed to be "remembered differently", or forgotten. Not in that particular respect anyway.
The thing about the TMP ship is that, backstage measurements aside, every part of her is bigger than the "original" TOS counterpart. The saucer could have been created by adding a new rim and bulging out the bulges a bit more; the secondary hull could have received a new surface layer of spaces; the neck could have been strengthened.
Basically, the old ship would fit inside the new one, save for the engines and the secondary hull bow.