This is a hack to work around the fact that we're using the wrong UI element for Discover's sidebar (see Bug 390928). We use a Kirigami GlobalDrawer for the sidebar, which is problematic because the GlobalDrawer's width is always a minimum of 360 pixels on the desktop--far too wide for our use case (see Bug 390927).
Replacing the GlobalDrawer with something more appropriate is going to be a lot of work, so until then, let's not torture our users with the humongous sidebar any longer and just force it to be a smaller size for now.
I think this is a vast visual improvement because it returns the focus to the content, rather than the navigation. It makes Discover feel less "fat."