From Team Fortress Wiki
The Backpack Expander is a tool item that appears as a backpack held together with four straps that has a Description Tag, Heavy Duty Rag, two Mann Co. Supply Crate Keys, and three Huntsman arrows protruding from it which also has a Sandman strapped on top. When used it will grant 100 extra slots, or two full pages, to the user's in-game backpack. Multiple Backpack Expanders can be used by the same player, but only up to a maximum of 20 uses to obtain up to 2,000 slots, which is the same as forty pages.
December 17, 2010 Patch (Australian Christmas)
- [Undocumented] Added the Backpack Expander.
November 21, 2012 Patch
- Increased the maximum size of backpacks from 1000 to 2000.
May 17, 2013 Patch
- Backpack expanders are on sale for 80% off.