Acording to this post "Fixed Width Editor Resizes With Zoom"
AmberV wrote:It is fixed to the width of the text, not the size of your screen.
tiho_d wrote:This is indeed by design and even reported as a bug previously. Scrivener for Mac and other editors work the same way. This ensures that your text fitted per line also stays the same until you hit the editor width.
That's fixed in single document view, but in scrivenings mode it's still maintain the old behavior.