This is an example page of course, but this is pretty clear what the issue is: LINK
Essentially I created a div, set width to 100px, and put a large (3200px w) image inside it. The image has variants all the way down to 500px. According to the responsive image docs and blog the designer should experiment with screen sizes and choose an appropriate sizes tag. But as you can see on the example page, it loads the image closest above 2x the viewport width.
What styles, elements, or attributes does the designer take into account when building the img sizes property? The only way I have been able to affect it at all is to put a fixed size directly on to the img tag.