So a smaller filesize AND a better quality on both screen types! […] The bottomline is that heavy compression doesn’t affect the final image as much as you would expect. This is because of the greater amount of pixels in the Retina image, compression artifacts are scaled down and therefore almost unnoticeable.
Via Hacker News, which questions the results. However, regardless of the precise file sizes, I imagine that the conclusion of JPEG compression beating upscaling will hold. There’s also a follow-up article (translated).