Hi SFP,
Some files cannot be compressed as they're already in a compressed format. No matter what sort of compression you take, that's as good as it will get. Zip files, certain image files etc. would be examples of files that won't compress at all.
Other files would, but the compression you would get is dependant on the files being backed up, and what your device is capable of. If you take an LTO3 tape, it's raw capacity is 400GB, while it's compressed capacity is 800GB. So it's 2:1, which is a best-guess scenario, and you will very seldom get it.
The best I can get is 1.5:1, but on other sites, the overall average is around 1.3:1.
Hope this helps?!?