On widescreen laptops, the maximum comfortable reading width is a lot narrower than the full width of the screen. The editor interface has a comfortable reading width which I would estimate around 40-50% of the interface:
But when you view the same page in the text view, you get a very different amount of space used and a very different reading feel.
Four comfortable lines turned into 1.3 very wide lines, and it also changes the vibe from “published article” to “prepress/typewriter/the formatting isn’t done yet,” which is ironic because that’s the public-facing view.
For the editor experience to more closely replicate what you’ll see in the final version, and for widescreen users’ reading experience to be less difficult, making the default body width about 50% of the full screen width would be both a kindness and an accessibility improvement.
(The menu width doesn’t have to be capped at the body width – the bar provides eye-guidelines to help people follow the thread across, and there usually isn’t a snap back at the end of that line.)