this seems to be introduced w/ the new version (great work to WP team btw). when you add an image & chose “full size” that’s bigger then 400px, the “width” is set to 400 no matter how big the image is (as in medium size insert).
it’s defaulting to what’s appropriate for your theme.
see if you can drag the corners of your image after you’ve loaded it to your post to bypass it. careful you don’t end up dropping your sidebar to the bottom of your blog in the process ;)