Featured image only 552 width

Today I found out that the featured images on a single page (like https://andersnoren.se/themes/garfunkel/the-story-of-love-locks/) are not in full resolution but only 552 pixel width (in the example from the demo: https://andersnoren.se/themes/garfunkel/wp-content/uploads/2014/06/love-locks1-552×345.jpg).
Why is this so? Was there a change in core code or in the theme?

Have a merry christmas
Manuel