Summary of both sides of this argument... I'm right, you're wrong, even though I have no proof.
We all know what a Galaxy
and a D'deridex
look like, and if we didn't before, this thread has pictures. Many of us probably own Micro Machines models of both, even. It's not rocket science to establish the volume of each ship (with Micro Machines or ERTL kits, you can do the Archimedes thing; with 3D computer models, you can just click a button), and then to scale the sizes up or down to see what really happens.
Here's a helpful page for a detailed analysis of starship volumes (excessively detailed for the purposes of this thread, but still):
The Romulan ship needs to be scaled down quite a bit from the 1300m estimate used in that chart to match the 5,800,000 cubic meter size of the Galaxy
... Essentially, even the 1000m version would be twice as bulky as the Federation ship.
Beyond this, it's speculation city, with material densities quite possibly well exceeding that of solid lead (that's the only way Kirk's old ship could approach Scotty's stated "nearly a million gross tons" mass). Starships are exotic beasts, and would be fully justified in packing exotic materials, including impossible ones; warp coils do impossible things, after all...