The huge loot bag is used in safecracking. It is obtained by upgrading a large loot bag with 5000 Pilfer Points at Dodgy Derek's Dirty Deals. When cracking safes across Gielinor, the loot will be added to the Huge loot bag. You cannot withdraw loot - instead, you have to fence it in the Thieves' Guild, with Robin.
A full huge loot bag is worth approximately 182,950-208,900 thieving experience, 294,750-393,250 coins and 676-757 Pilfer points when fenced.
Each safe fills between 0.5-1% of the huge loot bag. Commonly it takes about 102-198 safes to fill a huge loot bag. This equates to approximately 924-2048 experience, 1489-3855 coins and 3.4-7.4 Pilfer points per safe.
This is the highest tier looting bag available.