Still in Alpha times I've proposed large chests that are fit for two players to carry. Just imagine seeing a solo player digging out this huge thing (it would take much longer to dig it out alone) in panic and then very slowly dragging it to his sloop. In Gears of War there were levels where two players needed to cooperate to carry a very large chest. It was pretty fun.
I see it this way. 2 players can carry the large chest at high speed. One player can do it too but it will be veery slow. The chest doesn't need to be gigantic in appearance so it can still be mounted on the back of a climbing pirate. Twice the size is enough.
Every cursed chest can have a 2x size version with 2x stronger effect. That chest of sorrows...would be really deadly.. or just imagine trying to ballance out the chest of thousand grogs together with your mate.
Now that's the variety that would make a difference.