Looking at (..somewhat..) balanced freighters (the EI line), mass and cargo space mostly makes sense (there's a few bad values but nothing show-stopping): An EI500 has 200t of cargo space, with an empty mass of 150t. An EI100 has 250t of cargo space with an empty mass of 100t (kind of a bargain honestly). Finally in the light freighter lineup is the EI200 with 500t cargo space, and 500t empty mass (not a good ratio, but it works). Above that is the EI7000, with an empty mass of 2,100 tonnes, and a maximum cargo capacity of 2,000t (frankly that's an awful payload fraction but whatever).
The UAS T series of transports, comprising the T31 and the T55, are....somewhat interestingly TARDIS-like. The values flat out do not make sense given their sizes ingame (which may or may not match their XML sizes). Given that ship switching is increasingly a thing in both mods and future official content, egregious cases of bad values in ships that a player may well end up flying (like the low-cost UAS ships) should be addressed, hence my decision to ticket this matter:
T-31: Size 60 (150% that of an EI100). Empty mass 1000t (U wot mate?), 2000t cargo space. Given that the game has never modeled volume, this means you could carry an extremely compacted EI200 inside a T-31. In fact, you could carry four of them.
Recommendation: Drop mass to ~200 tonnes or less (it is an armored transport after all), reduce cargo space to 200-250t to make it an EI100-alike.
T-55: Size 95 (a bit bigger than an EI7000 but a lot narrower so I'm comparing it to the EI200 instead). Empty mass 1,500t (Actually that's about right for an EI7000 competitor). Cargo space: 5000 tonnes. The T55 can carry more than twice as much as an EI7000. Using the same "ignore volume" thing as before, a T55 can carry an Antares I.
Recommendation: Decide if this is supposed to be an EI200 or EI7000 competitor or (my favored option) an intermediary design, rebalance accordingly. I would suggest an empty mass of between 700 and 1,500 tonnes, and a cargo capacity of between 750 and 1,250 tonnes, depending on what niche you want the ship to fill when players inevitably get their hands on it.
A few notes:
- At 60 meters, the T-31 should probably mass around 600 tons. Assume the cargo volume is 50×20×10 meters, which is 10,000 cubic meters maximum. Maybe smaller to account for armor, etc. Water/ice is 1 ton per cubic meter, so it could be filled with up to 10,000 tons. But let's say that's too much because the engines can't handle that much mass. We probably still want cargo capacity of about 1,000 tons.
- The T-55 is 95 meters, or about 50% longer than a T-31. Since this is stretched along a single dimension, we can linearly scale it up. Maybe 900 tons empty and a cargo capacity of 1,500 tons. [These happen to match your suggested numbers.]
- Note that using similar techniques, I think we should scale up the masses/cargo capacities for the EI series. Not by much, but a little. The EI100 for example should probably mass around 350 to 400 tons and have a capacity of 500 or 600 tons. But I'm going to leave that to the future.
For 1.7 RC 1, I've changed the T-31 and T-55 as follows:
Ship Mass (empty) Cargo Space
T-31 600 tons 1,000 tons
T-55 900 tons 1,500 tons
That's a reasonable solution. I'll freely admit to not actually working out the cargo volumes. :P