Google’s article schema document has been update for AMP articles to require larger images in your markup. Before, the minimum requirement for your image sizes was 696 pixels wide and 30,00 pixels in total. Now, it’s 1,200 pixels wide and 800,000 pixels in total.
This is specific to the markup you use for your AMP articles so that they can appear in the Google search results top stories carousel. Currently, if you’re serving content and getting traffic from that carousel, you’ll need to make sure your images meet the new requirement.
The following screen shot shows the old requirements from the Articles schema developer document:
Now, check out the following screen shot of the new requirements that are live now:
Aaron Bradley is credited for spotting this.