Some contracts of bulky material don't make full use of ship cargo space

If you are trading something like Vehicle Parts II for Iron Ore, it makes sense that you don't need an entire cargo module worth of Vehicle Parts II to trade. However, it still uses all of the ship's modules worth of cargo; 1 module for the Vehicle Parts and the rest of the modules are filled with Iron Ore. However, for some contracts such as Slag => Sour Water, despite both products being a bulkier thing that makes sense to have a lot of, using an 8 module ship this contract uses exactly 7 modules worth of cargo. (2 for sour water, 5 for slag) I am almost certain there are other examples of this but I haven't looked through every contract.