I noticed the resizing has been disabled (or is that just me).
Now every image is being loaded fully scaled, which is very annoying to view at (have to zoom out to at least see most of the image) and takes a very long time to download due to the limited speed the Gelbooru servers are providing.
So does anyone know if this really is a bug or that it is something in my browser or settings?

You probably accidentally clicked the always show full size link, which is what causes this.

To fix it, clear your Gelbooru cookies and log back in.