tardis compressed tonnage i believe.
units make more sense. but it's too much work to have variable mass per commodity, and variable allotted space per tonnage.
It's why you can witchspace 1.5 cubic kilometers of expanded polystyrene in the comparatively small cargo hold of a cutter, just because it's 750 tons.
There you go it's like stuffing the City of London into something the size of a Cathedral.
Vorsprung durch handwavium at it's finest.
This was explained way back in 1984, though where I saw the explanation eludes me(what can I say, I was eleven). Tonne containers, in the context of the Elite galaxy are a standard shipping container size. In other words, a tonne container of feathers weighs alot less than a tonne container of mercury would. If you're having trouble not envisioning it based upon weight, remove the word "tonne" entirely.
The only game "wizardry" that a person is seeing here is that cargo isn't assigned a weight at all, which begs the question as to how much is actually in a tonne container in the first place. Do they vary as to how full they are? Are some packed like a bag of potato chips, and others stuffed to the top, then beaten flat to get the lid on? Where do all the fresh containers come from when mining?
On this, we don't get an answer, but yes, 100 "tonnes" of cargo does indeed take up exactly the same amount of space in your hold, regardless of whats inside the containers. And yes, I realize that this gets all the more confusing when a person also uses tonnes as a unit of weight measurement with their spacecraft itself.
As far as 2-4 tonnes of "gift" cargo space? No, I don't think so. I think we need to be making the hard decision to outfit for it or not, regardless of spacecraft. If the room exists on ships, however, I do think that Frontier could be tossing a size one or two slot onto existing craft that a person might find handy for such use.