Just curious how come some was able to
I don't think it's a case of "being able to". I suspect it varies dependent on the model/display size of iPad/iPhone in use.
On my iPad Pro 9.7", in landscape orientation, the preview pane / display width is 110mm / 198mm = 55.5%
On my iPhone 7+, in landscape orientation, the preview pane / display width is 73mm / 120mm = 60.8%
On both devices, in portrait orientation, the preview pane is 100% of the display width.
Maybe others with larger displays, i.e. 10.5. 11 and 12.9 would care to comment.
I cannot find any way of altering the size of the preview pane, it looks like you get what you get.
Maybe a good excuse for a bigger and better iPad.