If you have the original ISO file, you don't need to risk downloading shady "pre-compressed" files from forums. You can do it yourself using free tools. This method reduces file size by 40%–70% with no gameplay loss.

The PSP community is old and filled with dead links. When searching for compressed files:

Before looking for compressed versions, it is important to know which titles exist. The PSP hosted two major Batman games, both of which are highly sought after in compressed formats: