Issue Overview
Using the WP Retina plugin, a problem occurs where images smaller than the thumbnail (375px) do not display as Retina, even if a full-size Retina image is uploaded. This issue may be caused by incompatibility between the Responsive Image method of WordPress and WP Retina.
Solution.
To resolve this issue, you may want to change the WP Retina setting to “Picturefill method” The Picturefill method is a JavaScript library for providing appropriate images for different devices and screen sizes, and is used for Retina displays.
Changing the WP Retina setting to the Picturefill method will work properly with WordPress’ Responsive Image method to ensure that smaller sized images are displayed correctly as Retina.