A perfect site would look great everywhere, from a 34” ultrawide monitor browsing on a gigabit internet connection, to a 5 year old low end smartphone over 2G. Compression can help quite a bit, but at certain scales, it makes sense to fully resize an image. This could be as straightforward as a simple URL query flag, to a full blown image editor on a site builder. Whatever the method, it can lead to huge improvements in performance.
Testing was an examination of documentation for each service to see if resizing was an available feature. If it did exist, its functionality was confirmed.