Why doesn't ZIP Compression compress anything?

If you're compressing things that are already compressed (AVI, JPEG, MP3), you won't gain much other than packing everything in a single file.

The process of compressing takes repeatable patterns and tokenizes them to shorter patterns. The output is then mostly non-repeatable and therefore cannot be compressed by much, if at all.

"In a zip archive containing many files, each file is compressed independently. If there is a great deal of similarity between the files, then a different tool might give much better compression.

For example, tar.gz joins the files together, then compresses the results. Likewise a ""solid"" rar file makes use of similarities between files.

The downside of tar.gz or a solid rar is that you can no longer extract a single file from a large archive without decompressing the archive up to where the file you want is."