In the ROM and emulation world, "highly compressed" refers to reducing a game file size using advanced algorithms (like CSO compression for PSP). A standard Sims 3 ISO might be 1.5 GB. A "highly compressed" version can shrink that down to without removing core gameplay.
The Sims 3 is a life simulation video game developed by The Sims Studio and published by Electronic Arts (EA). The game was initially released for Microsoft Windows and OS X in 2009. Later, a PSP version was released, allowing users to play the game on their portable device. However, the PSP version was not widely available due to its large file size. To address this issue, highly compressed ISO files of the game became popular among gamers. sims 3 psp iso highly compressed