This question came over me when I tried to fill my ship with some raw materials.
In fact ram materials take much mor tons than m³ but nearly every ship has more free m³ than tons. I can think of a need for it as long as the ship has hangar bays or docking bays but it also applys to every small freighter.
What I want to ask: is there something I can do with my spare m³? Are there some equipment, I can add to my ship? Maybe some sort of better (and stronger) engines to raise the weight limit while reduzing the m³ limit (because the extra engines take up space)?
at the moment you can´t add any modules or upgrades to your ship. Will be possible in the future and then they will ofcourse take up space depending on the type and quality of the upgrade.