The Large Weight Sails increase the carry weight of the ship it is placed on.
It weighs 100.
|Large Weight Sail|
The "Sail Max Movement Weight" stat that comes from blueprints gives a percentage based bonus to the weight. The bonus calculation is based on the base value of the sail and not the ship.
A large weight sail with a "Sail Max Movement Weight" bonus of 110% will be calculated as 4000 * 1.10 = 4400 weight for that sail.