Iv done a bot of looking around, and i think at least the reason for the sterns being narrower and higher at least, was to help deter boarding, and in the event of close ship combat, to give archers and later musketeers a good platform to shoot from. It would seem that through out the 17th century as cannon(guns) became more in use, the stern and forecastles were lowered as the risk of boarding was reduced. I also believe that the curved shape at least in part may have helped in draining and preventing pooping, but i still like the gravity idea as well lol