If you set the ruler in the normal edit view if affects the width of the text in fullscreen mode in a way that doesn’t mirror how it you set it in normal view.
For the above screen I have the right tab at 18 cm, of 33cm in normal view.
When I switch to fullscreen mode, the text is much narrower. In full screen mode, the tect covers roughly 1/3 of the availible “sheet”.
(and now you can all see how I wrote in 1st grade with those wonderful green crayons…)