Hello Tom,
thank you very much.
The second snippet works.
I now consistently get small images on archive pages, and as far as I can see, srcset on individual posts.
I understand that for individual posts it needs to serve a size that can satisfy both vertical and horizontal display.
What still slightly confuses me is this:
When srcset is active, a specific size is specified, the image is available in that size, then still a larger image is being served and resized to the specified size.
If srcset works in this fashion, I think thumbnails/fixed-size images are best treated differently from normal post images.
I guess these large magazine sites don’t use srcset to get the size down for all these preview images.
And many thanks also to David for looking into this.
Have a great day.