side-by-side diff does not use available horizontal space

When viewing a side-by-side diff, there is great swathe of blank space to the right (about 1/3 of the available space) which is not used. That means that, particularly when I'm zoomed in a little to make the text font size readable, the text wraps much more than it should (in the case I'm looking at, around 67 characters) which makes the code hard to read.