I've found recently, both on Wikipedia and my own wiki, that 2x resolution images are not being delivered by MobileFrontend/MinervaNeue. If I download an image from an article on Wikipedia in the mobile skin, then switch to the desktop skin and download the same image, the new image is double the resolution.
Topic on Skin talk:Minerva Neue
This is intentional. I think https://phabricator.wikimedia.org/T119797 is the associated Phabricator ticket.
Note this should only impact mobile views. Minerva without MobileFrontend should be fine.
Thank you! From that thread I found this one, and I've managed to re-enable the functionality by setting $wgMFStripResponsiveImages to false.