[qntm]

Suppose the random file is 3,000,000 bytes long and happens to contain 135 bytes "X". Split the file on the letter "X" into 136 smaller files, each with no X in it. Then, return the files along with a "decompressor" script which says something like:

    return(join("X", files 1 to 136))

Since the script takes up fewer bytes than the 136 "X" bytes would, the total size of the 136 files + decompressor script is less than the size of the original file.