According to John Eaves (or at least, implied in the way he talked about it), the intent was so that the Enterprise would be "special" and stand apart from all other ships, as it would be the only one with cylindrical nacelles. It was also intended that most other Federation ships would have much "flatter" profiles than the relatively tall Enterprise, again as a way of making the Constitution class seem unique and separate from the rest of the fleet. This suggests that the idea was to make the Enterprise stand out from the rest of the fleet without something so banal is just making it "bigger" or flashier or something, that it would feel like an entirely different TYPE of starship compared to all of the others.We know who gave the starship design direction at least, Bryan Fuller.
I don't remember if the 'why' was given.
As for the ships themselves, that's pretty much just John Eaves being John Eaves. I find alot of his designs to be sort of basic, and I suspect that most of this "It doesn't fit the 23rd century aesthetic!" nonsense is just people trying to justify the designs seeming kind of boring and uninspired compared to what we were expecting.