I bought some Ship Hull Parts from Leeds, came to sell them in Freiberg, Stuttgart and I realised my cargo (600) was full with only 300 Ship Hull Parts. I then bought some Vacationers from Baden Baden and noticed the same thing - I can only take 300.
Why are some items 2 cargo spaces, and which items are?
That's intentional - some commodities, like for example passengers, require additional cargo space per passenger to accomodate the modules that carry the passengers themselves, or for their luggage.