So where is your proof that it is bigger in mass?
The proof that it is bigger in volume is right in front of your eyes. If you want to insist that it is built out of lighter materials, feel free - but IMHO that is utterly idiotic.
FWIW, the DS9 Tech Manual mass figures are based on a fundamental error in thinking: they are proportional to the length of the starship. But starships are not cigar-shaped, and the mass should be proportional to at least the square of the length if not the cube...