The small loot bag is used in safecracking. When cracking safes across Gielinor, the loot will be added to the small loot bag. You cannot withdraw loot - instead, you have to fence it in the Thieves' Guild, with Robin.
A full small loot bag is worth approximately 16,850-29,250 thieving experience, 24,750-55,000 coins and 63-92 Pilfer points when fenced.
Each safe fills either 6, 8 or 10% of the small loot bag. Commonly it takes about 11-16 safes to fill a small loot bag. This equates to approximately 1053-2659 experience, 1547-5000 coins and 3.9-8.4 Pilfer points per safe.