Comment by Hendrikto
6 days ago
Some social media sites and apps like to cut off the sides to fit content onto different screen sizes and aspect ratios.
I think adding a border is an attempt to preserve the essential parts of the images in those situations. It really should not be necessary, but alas modern/stupid problems require modern/stupid solutions.
This is a more generous explanation than my cynical first assumption, that they were trying to get away with saving lower res pictures to save on storage costs.
It's optional you can turn it off at bottom floating navbar.