Well, I agree that the deuterium tanks are big. And a cubic centimeter of deuterium might even be heavier than a cubic centimeter hydrogen.- so, there is certainly a lot of mass there. But think about all that metal and what not resting in saucer. In the saucer, more room is filled with denser matter. The battle section would probably fit 2-3 times in the saucer volume!
I don't agree with the deflector-point. We don't know much about the materials used for it ( FC states its loaded wit anti-protons - not quite heavy stuff.). But the saucer has enough comparable architecture 3 or 4 time the volume of the deflector dish. On the contrary: I'd even state, that the deflector dish takes away from the potential to have more mass in the battle section.
Considering the nacelles... it's a little weird to count them in. The point is, the volume and the mass of the nacelles aren't really important, because we're talking about how big the nacalles should be to carry a certain mass.
If we can make one point then we would have to say that the nacelles could be even smaller, if just used with the battle section.
But don't let us get drawn away! The point was that you need to give up space in the saucer section, if the saucer is supposed to have warp drive. But, in the eyes of starfleet, this could have rendered the ship useless to them. It's still a scientific vessel.