What's the rational for Apple behind the width of this feature? In desktop vs iOs devices? Is it always the same or dynamic base on proportion?
The original Readability codebase is embedded in a host of applications, including Apple's Safari 5 browser (the Safari Reader feature), ...
The Read Now and Read Later bookmarklets (found here) use the same width when transforming pages into more readable articles. But how they made the decisions on width, font face and size can only be guessed since the project originated as a Arc90 Lab experiment from which the website is down.
If you try out the bookmarklets in a desktop browser and resize the window, you will see that the page is responsive. I noticed besides the normal font size (21px) that there is a smaller font size (19px) on small view ports (around <700px) and larger font size (27px) on view ports larger than around 1600px. I guess the same applies for Safari: On an iPhone 5 (640px width) this would give the smaller font size and on an iPhone 6 (750px width) the normal font size. I based this on my observations on an iMac so please correct me if I'm wrong.
A history of Readability can be found on their blog.