Some weeks ago, OP apparently adjusted the cargo restrictions on several types of cargo, making a number of trades & mining much more attractive financially. High-value cargo still usually takes more cargo space than normal goods, but the restriction now appears to be 2:3 instead of the previous 1:2 ratio for items like diamonds, niobium, etc. Other goods have been relaxed much more, especially raw materials such as water, alien organisms and scrap metal, which are now 2:1 compared to normal cargo volumes. This means that mining for raw materials got twice as profitable, since the value of a full cargo hold has doubled for these goods, and mined commodities are free.
I have still not figured out what the reason for the various ratios may be, apart from putting some controls on the rate at which a player can build wealth through trading and mining. I once thought that the space limitations had to do with the maximum mass a given cargo hold would be capable of carrying, but OP insists that cargo mass does not affect ship performance, so only the value could be controlling the reduced ratio of space for diamonds.
Either way, these changes are good news for law-abiding traders and miners, who now can look forward to better compensation for the risk of predation on their legitimate businesses!