DU resized all uploaded images down to a maximum width of 300 pixels.
The 4K screens as well as most phones and tablets, can use double that size in the same space, to offer a sharper image. As of today, DU will use a maximum width of 600 pixels.
If you upload small images, below 600 pixels width, you may not see much/any diference.
It seems to be working. The difference on a normal HD laptop/desktop will be nearly impossible to tell, even though the new smaller version has been improved too. The difference is best seen with a phone/tablet or 4K screen.