Today, 5:45 AM · 32 replies and 599 views.
You can't work out the mass from a volume in a space ship. It's not a block of a homogeneous material where you could just put a volume and density into a formula.
A ship can be more or less hollow, depending on her purpose and the bulkheads and hull armour can be of a various thickness.
For example Dropship and AspX are basically of a same size, but dropship simply has the armour twice as thick, so she's twice as heavy.
That being said, I do agree that generally the masses of several ships are way off. This is an artificial problem Frontier "created" for (or rather against) themselves. The modules have a given performance (generally, too, but now I'm talking specifically about thrusters and FSD) and FD can't do anything with them. If they did, they would throw all the ship that use said class of module off balance. The only way they can "balance" a ship to their intentions is by adding and subtracting mass.