Large images

My wordpress settings are set to generate large images at max width 1440, max height 1024.
But in WP Retina it says: “The full-size images should have a resolution of 6000×3000 at least for the plugin to be able generate the…”
How come I need 6000×3000 when my largest size is 1440?
Also my 1440 size it not generated to Retina.

https://ww.wp.xz.cn/plugins/wp-retina-2x/