I think there is a CSS rule here that limits the width of #main-outlet. I suspect it’s in your custom theme. When I use Horizon (this is based on Horizon, yes?), I don’t see the same issue.
I am using Horizon, sorry I forgot to mention that. I was limiting the main-outlet width to make room to nudge the sidebar over so it wasn’t hugging the edge of the screen.
I reverted those changes so now the login screen takes up 100% of the screen like it’s supposed to. Sidebar has to touch the edge of the screen now but that’s not nearly as bad of an issue.