Compression ratio depends just as much on the type of the fyle to be compressed as it does on the program you use to compress the file.
You can often get 7-1 ratio on text files, executable files and such but not on file formats like jpg, mpg, gif, mp3 and other types of media. That is because files in these file formats are already compressed. There is nothing left to compress there 
In case you are interested, zip compression is working using Lempel-Ziv algorithm. Basically it works by finding many possible repeated subsequences and substituting them by shorter subsequences.
Here is the animation of how it works:
http://www.data-compression.com/lempelziv.html
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My Blog: durafei.blogspot.com - Last Update March 23, 2006
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