O.k. Pilou, i checked the --help entry and Guetzli defaults to 95%...

With 100% it would be 476.217 Byte
and with 90%, like you did, it would be 84.770 Byte.

I'm curious now what you would have with 95%?

You must have a very good encoder for Windows! :-)

P.S. just found ImageOptim for Mac, wich contains
Zopfli too. https://imageoptim.com/mac

Regards
Stefan