Firstly I rule that only 1 cargo hold type of component (excluding the barracks) can be fitted to each ship unless it is built in - such as the Star Galleon that has 2 main cargo holds and the Mass Conveyer with 4. This prevents the crazy ship full of main cargo holds builds which are valid by RAW.

Next I use multipliers based on ship size class because 1 space on a frigate does not equal 1 space on a huge Mass Conveyor.

Frigate/Raider x1

Trader/Cruiser(incl. Light) x2

Grand Cruiser/Battleship x3

Mass Conveyer/Goliath x4

So for example a Main Cargo hold can theoretically add 125 AP to a Trade endeavour for a Frigate, 250AP for a Trader, 375 for a Battleship or 500 on a Goliath.

The counterpoint rule to prevent abuse is that this AP is only added if you can fill said holds.

Easy enough to fill the hold of a frigate even in the Koronus Expanse most of the time but good luck filling a Mass Conveyor on a random stop.

Larger craft would need to have pre-planned destinations that have centralised transport facilities and massive warehouses where whole planetary harvests would be brought to one craft.

The ships with the largest class of hulls would often travel with a partial load and due to their specialist natures would need escort craft to protect them which would eat into profits.