Maybe it's not intended to work this way, but on my Drupal 7 site, I had the "Maximum image insert width" set to 650 pixels. When I would insert an image, say that was 800 or 1000 pixels wide, the inserted markup would've scaled it to be 650 pixels wide (but not using an image style, still linking to the original image), and proportionately scaled the height.
It seems with the Drupal 8 version, when I insert an original image with that setting:

Then I insert an image that's 1200 pixels, the image's width is set to the original image size and not scaled proportionally so the width is 650 pixels:

Comments
Comment #2
adamps commentedI can't find the code so I think it's missing entirely. Maybe should be in template_preprocess_insert_image?
Comment #3
snater commentedSeems like the setting was indeed not considered in the code… The attached patch should fix the issue, hopefully. :)
Comment #4
adamps commentedThanks @Snater that works for me.
Comment #6
neslee canil pintoComment #7
adamps commentedGreat thanks for the commit
Comment #9
geerlingguy commentedJust wanted to note that I can also confirm the patch and new release fixed it for me. Sorry for the late update :D