The medium loot bag is used in safecracking. It is obtained by upgrading a small loot bag with 500 Pilfer Points at Dodgy Derek's Dirty Deals. When cracking safes across Gielinor, the loot will be added to the medium loot bag. You cannot withdraw loot - instead, you have to fence it in the Thieves' Guild, with Robin.
A full medium loot bag is worth approximately 34,900-46,400 thieving experience, 54,500-96,750 coins and 128-171 Pilfer points when fenced.
Each safe fills between 3-5% of the medium loot bag. Commonly it takes about 21-32 safes to fill a medium loot bag. This equates to approximately 1091-2210 experience, 1703-4607 coins and 4-8.1 Pilfer points per safe.
A Medium Loot Bag can be upgraded into a Large loot bag in Dodgy Derek's Reward Shop with 2,000 Pilfer Points.