This is not at all a surprising outcome. The result of the very redundant input file is (as a result of the limited range length of the compressors and the fac

This is not at all a surprising outcome. The result of the very redundant input file is (as a result of the limited range length of the compressors and the fact that they output chunks of literal data) a smaller, but still somewhat redundant output file. The subsequent passes are simply operating upon the redundancy created by the compression algorithm itself. Try compressing a file of nulls and look at the output with hexdump or a hex editor. You'll see the patterns left over by the compression algorithm: dd if=/dev/zero of=test bs=1024 count=102400 gzip -9v test "look at contents of 'test.gz'" -- lots of redundancy gzip -9v < test.gz > test.gz.gz "look at contents of 'test.gz.gz'" -- looks more random