Google has updated its article schema document for AMP articles to require larger images in your markup. Previously, the minimum requirement for your image sizes was 696 pixels wide and 300,000 pixels in total, but now it is 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. If you are currently serving content and getting traffic from that carousel, you may want to make sure your images meet these new requirements.
Here is a screen shot of the old requirements from the Articles schema developer document:
Here is a screen shot of the new requirements that are live now:
Hat tip to Aaron Bradley for spotting this.