Why do you want to find out how heavy certain ships are?
If I know a ships (relative!) weight, mass or volume I can use that as a basis to calculate damp, rotdamp, max velocity, rotation time, acceleration, explosion radius, explosion damage, explosion blast, primary and secondary capacities, power output which in turn affects shield strength and shields and weapons regen, hitpoints, scan time and score. Probably more too and each of those can be used to calculate other values.
Then you can add modifiers wherever you see fit, for example to distinguish Fenris, Leviathan and Cain and Lilith from each other. Or tech level modifiers to make FS2-era ships better from FS1 era ships.
Bounding box volume allows me to do all that, with one caveat. A ship may be of a shape where bounding box size does not reflect the ship's actual volume, or even close enough. One such example would be the Seraphim. Eyeballing a modifier into the volume calculation would be stupid, because other ships have similar issues in different scale, such as Fenris/Leviathan (radar dome and missile turret).
So unless some bright coder can add somewhat accurate volume calculation into PCS2, the bounding box volume seems be the only option here. Unless somebody knows how to use Blender to calculate model's volume, mass or weight. I tried to comb Blender's confusing UI to find whether it could do just that, but I didn't find such option. Googling didn't reveal anything either, oddly enough. Which seems to indicate such feature does not exist in Blender.
I already have a spreadsheet including all FS1 ships where I've used bounding box volume as a base to calculate all other ship stats. It does provide very nice variety for even fighter sized ships.