I am working on upgrading all of the wallpapers to 4K resolution since it's more sought after. I can easily do the all new extras in 4K, but I had started with 1080 size due to it being something I work with and design for a lot for stream graphics.
I have tested some 4K upscaling for the digital art wallpaper series that I done to get things started on here. In some tests if around a 1000 zoom level you can see a slight difference and start seeing quality loss. It's so miniscule that I personally think they would be fine upgraded to 4K.
For now If you want the 1080 original size of these with your purchase just let me know and I'll get it to you asap. I am going to be testing zip files of multiple resolution sizes. If it works, or I can find a way to get it to work. I'll replace all of the current extras with them instead of just the images.