A CSO (Compressed ISO) file is a compressed version of a standard PSP ISO image. It was developed to help gamers save space on their Memory Sticks, which were notoriously expensive and limited in capacity during the PSP's peak years. CSOs can be 30% to 60% smaller than ISOs. Format: It uses the DAX compression method.
While the gameplay content is identical, using compressed files offers several advantages for the modern user:
Connect your PSP to a PC and locate the ISO folder in the root of the Memory Stick.