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 🙂
This error will show up in Google Search Console (AKA Webmaster Tools or WMT) if there is an error with some or all of your AMP pages.
Here’s an error which has started showing up for a lot of stores’ product category pages – WooCommerce stores in particular – in Google Search Console:
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.