Why would a space ship use a wet ship measurement?
I mean, it's very sensible for ships meant to float on water to weigh themselves equal to how much water they can safely displace without sinking, but in space?
In space it's far more important to weigh your actual mass, because that has far more influence on your ship's performance profile and structural stress than volume will.
Because we needed (well, wanted) a way to conceptualize the actual dimensions of our ships. Originally, Aurora just did everything in Hull Spaces, and what that actually equated to was left entirely to the player to decide. We agitated for a more defined number, and various volume-based suggestions were tossed about (for example, the idea that 1 Hull Space = 10m x 10m x 10m was popular) but for thematic equivalency to wet navies, displacement won out. Then the argument was water displacement versus air displacement (since at the time Aurora ships were atmospheric-and-exoatmospheric) but we nigh-unanimously agreed air displacement gave silly-sounding numbers and water displacement kept the 'real navy' feel. Steve spent an enjoyable afternoon researching all the ways real world displacements are measured and/or calculated, and the vocal majority of us (or, at least, the majority of the vocal us) agreed on 1 Hull Space = the (calculated) volume required to displace 50 tons of pure H2O at 20C & 1 Atm.
In Aurora, a ship's mass actually
doesn't matter, as shown by the identical performance of an empty freighter and that same freighter hauling a entire Research Complex. Or that of 57,000 ton supercarrier fully loaded, or empty of its 30,000 ton strike wing. The technobabble explanation is that TN engines push the ship through the "Luminiferous AEther" - and it's the size of the 'bubble' that matters, not the mass inside it.
Sure, in the real world every kilogram is so critically important that some nations send their astronauts up on an empty stomach (or maybe that's just a vomit-related safety precaution) and the Space Shuttle had a not-quite 7% payload fraction to get to Low Earth Orbit. In Aurora, TNE changes everything.
So, to summarize, I respectfully disagree with your contention that "Naval guns at their smallest are so large they can't reasonably be lugged around by anything that has to operate in a ground combat context. . . The smallest laser without reduction technology is 150 tons or so. That should be super heavy unit range."