I made a really dumb mistake. An image was really tiny on my monitor, so I went to click "Show Original", but accidentally hit "Always Show Original" instead. At least, that's what I think happened? Now every image is huge and I can't figure out how to fix it lol. Maybe the issue is that I'm on Firefox? I'm clicking the "Resize" button, but nothing happens.

Go to options in your account page. There's an option for changing that behaviour.