the width of the content section is 540px (narrow), 720px (Standard) and 864px (wide) (and 920px for tablet size).
There is an special reason for such size?
I think is neesary to have a relation of the crop sizes of the images (now: 300px and 1024px), the queries jumps (992px, 768px, 480px, 320px) and the webbok width sizes (now: 540px, 720px, 864px, 920px). Otherwise, the user is having a non optimice image.
With gutenberg we will be able to have more images and better organice. So, is good to start to have a clear sizes aproach.
Users would have one big image or two images in the same row. The image would be the big one or a litle version if they open from a mobile. So, if we have a clear webbook sizes and clear images crop sizes, it would be much more optimiced.