Ironically, the avif demo image they use in this write-up is larger then the failover jpg (137kb vs 110kb).
https://reachlightspeed.com/img/blog/post-using-avif-images-...
https://reachlightspeed.com/img/blog/post-using-avif-images-...
https://reachlightspeed.com/img/blog/post-using-avif-images-...
https://reachlightspeed.com/img/blog/post-using-avif-images-...
The JPEG file is 112kb and the .avif file is now 55kb.