This is used to provide higher resolution thumbnails for high pixel
density displays, such as phones or laptops. If your screen has a 2x
pixel density ratio, then 360x360 thumbnails will be rendered at 720x720
resolution.
We use WebP here because it's about 15% smaller than the equivalent
JPEG, and because if a device has a high enough pixel density to use
this, then it probably supports WebP.
720x720 thumbnails average about 36kb in size, compared to 20.35kb for
360x360 thumbnails and 7.55kb for 180x180 thumbnails.
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