Check out this result of debugging an article using Facebook debugger.
It comes up with this warning:
og:image should be larger. Provided og:
what did i notice when i run into Scraped URL is that facebook is not able to see the picture at the same size like in the posts. For instance, copy the url of the picture which in Debug is to small, paste in to a browser and indeed the picture is very small. Now, go to your post, copy the url picture from your post, and paste in to the same or any browser. Now the size of the picture is big.