From Guild Wars 2 Wiki
Double-click to consume.
Double-click to increase the maximum stack size of all bank collections by 250.
— In-game description
- A maximum of 3 Collection Expanders may be purchased per account, for a maximum stack size of 1,000.
- This only applies to the collections tab in the bank. Moving a stack with more than 250 items in the collections tab out of the collections tab will result in either several smaller stacks or only 250 items withdrawn.