A "highly compressed" file implies that sophisticated compression software (like WinRAR or 7Zip) has been used to shrink the game file down to a fraction of its original size, such as 50MB, to make it easier to download and share.
The legality of downloading and playing highly compressed games can be ambiguous. Many games are copyrighted, and downloading compressed versions without owning the game or obtaining it from an authorized source could infringe on copyright laws. Pes Psp Highly Compressed 50mb
are sometimes used, which offer slightly better compression ratios than standard CSO. Performance and Compatibility such as 50MB