The app should not load higher resolution images when on a metered connection.
The patch I created stops images from being widened on metered connections and ensures they're still centered properly when not widened, but there is actually more to this task when it comes to hi-res images. We have code in widenImages.js, mainly for the benefit of older phones/browsers, that parses out and grabs the highest-resolution images from any srcset attribute it finds when the image is being widened, since srcset was introduced in HTML5 and isn't universally "known" to browsers in the wild. So on older devices, the hi-res thumbnails won't be downloaded. Newer browsers, however (including, I think, the system webview on Android Lollipop+) opportunistically grab the "best" (usually highest-quality) image based on the srcset attribute without any developer intervention. On these platforms the app will grab a higher-res thumbnail even if image widening is disabled.
We no longer serve HiDPI thumbs on mobile web at all; as a product question, is this a step we want to take in the Android app as well? Varying the presence of srcset attributes in the response based on the user's connection state would have negative response caching consequences that it's not obvious to me how we'd get around (unless we wanted to start having multiple endpoints to serve content with/without srcset attributes, which doesn't seem worth it). So I think we likely have to go one way or the other on this.