Almost all CDNs/services compress images lightly if at all, offline tools are better at this, e.g. images from the reference page can be ~20% smaller. Ideally, hidpi images should be compressed harder, e.g. lower JPEG quality.
Hopefully talking about the 'why' will help others make the right choices for their project.
The reference page will see improvements in the future as we enhance our process, but certainly resizing and optimizing variants by hand can yield even better results if you have the time.
What is worth noting from part 1 is that using pixel width variants in `srcset` will let the browser factor in dpi of your monitor as part of the calculation. So it wouldn't serve a 2x image where it only needed 1x.
It's not recommended to use high dpi images, 72dpi is the way to go for web content, with a larger (dimensions) version serving as the 2x variant.