python zipfile module doesn't seem to be compressing my files

I made a little helper function:

import zipfile

def main(archive_list=[],zfilename='default.zip'):
    print zfilename
    zout = zipfile.ZipFile(zfilename, "w")
    for fname in archive_list:
        print "writing: ", fname
        zout.write(fname)
    zout.close()

if __name__ == '__main__':
    main()  

The problem is that all my files are NOT being COMPRESSED! The files are the same size and, effectively, just the extension is being change to ".zip" (from ".xls" in this case).

I'm running python 2.5 on winXP sp2.