Good catch.
You likely won’t notice a lower resolution image on smartphones due to their screen size, and I think, in general, they are more likely to have a slower Internet connexion via the mobile network than a home Internet connexion. In this regard, I wouldn’t expect a mobile experience to load a larger image than on a desktop computer.
But scrset dictates which image to load for which pixel ratio. 
It’s definitely not a bug, I’ll move it back to Contribute > UX 