If you save this image on your desktop, you see it's background color as your desktop color. If you open it with Adobe Photoshop, its background id checkered which means it has no background. It's transparent.
The background data is removed and I think this image has this size without any compression because no lag in zooming compared to the same image with white background (you can find it through trial and error and converting it to different formats with different compressions) which leads to faster scrolling speed when converted to PDF. How we can do it?
How to produce a 1-bit transparent gif?
By the way, in archive.org it's recognized as animated gif.