compression works best with text based files and pictures due to the repeats in the file itself, ie removing all the repeating words and replacing them with a number. the numbers are then put into a type of dictionary that would decipher the compression when decompressed.
as far as compressing things like mp3s or mpegs, you wont get much compression with them, i just zipped 382 megs of mp3s to zip and the resulting file was 377megs. A 5.5 meg document file was compressed to 350kb.
best thing to do would be to back up files to disc like antman said or delete all your pr0n 😛