This is not about the visual editor for mobile but the source editor for mobile. Please let me know if this is not the right place for this piece of feedback.
I use Wikipedia on mobile via Firefox for Android on a device which has a small screen size (320x452 browser window size).
As the screen size is small, the space I get is limited already. Besides that, the mobile source editor seems to be wasting precious editing space by having lot of unnecessary white space surrounding the editor. See linked screenshot. As you could see, effectively the available editing space is only half the screen size! It would be nice if the unnecessary white space was removed used to give more editing space.