Umm, what? Clearly, the volume of the warbird is much greater than that of the Galaxy
if we accept the external dimensions shown above.
Eyeballing from the pictures, a single Romulan nacelle is already worth three Federation ones (and let's remember that the makers of the DS9 Tech Manual from which these figures come assume that some 25% of a starship's mass is in her warp coils.). The saucer is but a fraction of the size of either the upper or
the lower wing. And then
comes the beak section of the Romulan ship...
might be a tad less massive/voluminous than the Galaxy
if both were of equal length, yes. But it would be a close call even in that case.
If we take this Tech Manual set of figures, we probably have to assume that the analyst let a digit out of the Romulan mass figures... 1
4,300,000 metric tons is closer to what the volume ratio suggests!