I’ve had to explain this a few times recently, so probably there isn’t a good, clear explanation out there on Google, for this little corner of Google 🙂
Google Webmaster Tools
New Google AMP Warning “Image size smaller than recommended size”
If you’re running AMP for fast loading pages on the mobile web (and, if you’re not, you should give it a try, it’s awesome!), you maybe got a notification in Search Console (AKA Google Webmaster Tools) that “Image size smaller than recommended size” for some or all of your AMP content.