But... In many environments, you're going to see both. There are going to be JPG files which have already been compressed, as well as other files that do not compress down like zips because they're already compressed. This files sit there right along side text, docs, html, and others that will zing down to next to nothing.
Your best test maybe to do a file copy and monitor the start and stop time of your copy. That will be better than any artificial test.
John