Ah! I think there may be a configuration issue in your install. We also use a GZIPOutputStream for gzip compress in Booster so should be getting the same compression level. I believe the issue may be that you have not configured Booster to compress "application/vnd.ms-excel" content types. Try (in puakma.config):
Also ensure BOOSTERNoCompressTypes= is either commented out (disabled) or does not include the excel types.