This idea is not to change any of the existing games, which are already excellent, but for a new game that Vice might or might not be developing (or planning) ;-)
Augmenting the physics model slightly, increase the ship size variance (the size variance would be proportional to the mass variance) and so larger ships would naturally be more sluggish, unless they have more massive engines.
However, instead of equipment slots, there would be space and mass considerations for having extra equipment. Hence, if you have a big ship, you have more room for equipment. However, the bigger ship requires bigger engines. Larger ships and larger engines are also more expensive. They are also larger targets (i.e. if you're in a big ship, you would be more likely targeted by enemy AI, such as pirates).
For every piece of additional equipment you add, the ship would take on more mass, and thus would perform more sluggishly. This would translate into longer turning times and also longer acceleration times (though the top speed should be unaffected: it just takes longer to get to it).
The same would go for cargo. If you have an empty cargo hold, your ship would perform much better than when you have sucked up a full hold of platinum, at which point your ship would be quite sluggish.
