I think that weight wouldn’t be an issue however as you put it that 1m3 of rice is roughly 781kg.
However a standard fit pallet is 1000mm x 1200mm x 120mm and weighs ~25kg. Now as a general rule of thumb, a pallet should not be loaded higher than 1380mm not including the height of the pallet at 120mm.
However, one pallet described above varies in weight depending on what it is carrying. So that implies the pallet is as a whole is 1.8m3 but carrying flour would be about 1,200kg. A 20’ container holds 10 of those pallets equating to a net weight of 18,000kg (20,200kg gross).
If it was a pallet of say, mining machinery, you will not fully use the “1m3” in a generalised cube. Tractor parts come on pallets and if to help understand, 1m3 is not always a cube. Sometimes it is 600mm wide by 2000mm long by 300mm high is 1.2m3 and weighing on average 2,200kg.
So unless there is a factual representation of how it is in game, I think it might be safer to say that there is a genrelized unit of measurement that suits the developed logistics industry in game. Because weight don’t matter and so you can fit 1m3 full of feathers or bricks, it will stay the same volume. Unless you’re buying Hull segments in the 10’s of thousands.... then **** you devs!!! (Kidding)
But looking at it, I would summarise that the cargo size is perhaps measured in volume over weigh, and I’d haphazard a guess to say that 1 cargo space could essentially be 10m3, or even 100m3. Some of those ships are massive....
FYI:
Average container ship berthing in nz carries between 1000-1500 20’ and 40’ containers averaging about 15,000kg. These are 250-400mtr long vessels and to compare to say a Mastadon, then the volume is all messed up in game,
Summary, don’t boggle your mind with something as trivial as actual numbers in a completely fictional universe. It’s just not worth the energy.