Fuzzy image display on new content type
Based on the markup on your test page reflecting an html5  element and the code snippet calling for a "picture_sizes_formatter", I suspect your site has the Picture and Breakpoints module) installed which automatically serve images based on predefined/configured breakpoints.  

Go to the Picture Mappings configuration page Config >
 Media > Picture Mappings (/admin/config/media/picture) to examine existing mappings and add new ones.


You should be able to add a new picture mapping for a wider breakpoint to serve the original image, thus limiting serving the large image to a narrower desktop viewport.

Once you've added an additional mapping (and possibly a wide breakpoint), don't forget to clear the caches before reloading your test page.