The large loot bag is used in safecracking. It is obtained by upgrading a medium loot bag with 2000 Pilfer Points at Dodgy Derek's Dirty Deals. When cracking safes across Gielinor, the loot will be added to the large loot bag. You cannot withdraw loot - instead, you have to fence it in the Thieves' Guild, with Robin.
A full large loot bag is worth approximately 95,350-112,350 thieving experience, 157,500-225,250 coins and 350-405 Pilfer points when fenced.
Each safe fills between 1-2% of the large loot bag. Commonly it takes about 52-98 safes to fill a large loot bag. This equates to approximately 973-2161 experience, 1607-4332 coins and 3.6-7.8 Pilfer points per safe.
A Large Loot Bag can be upgraded into a Huge loot bag in Dodgy Derek's Reward Shop with 5,000 Pilfer Points.