Why is 50% and 20% larger than 80% when exporting a file?

I noticed this while exporting a file today: (30.8.1, macOS 26.2)

Why is 50% and 20% larger than 80%?
In this case I’m exporting vector art, but I also tried it with a photo and the same thing happened.

Not only is the file size smaller in save for web, but also looks better when zoomed in. Legacy compression for the win.