the featured images of the blog posts are shown in full size - I guess since last WP update.
If I add a new pictue to WP media this will be shown in full size since some days or weeks. If a use a picture I added some time ago, this will be shown correctly in size of 670*300.
You can have a look at my website https://fruchtstueck.de/?cat=170. First blog entry with big, second with normal size.
I tried to check if the uploaded pictures are corrupted: no! It is reproducable.
Having a look at the file system I can see that recently uploaded pictures were automatically coverted to more preview formats than before. My feeling is that this confuses the travelify theme. Have a look at the screenshot with comparison of picture files. First one has wrong size. Second is fine. The pictures are related to mentioned website with the post #22 and post test (at end).
What is your impression? Can you help?