If you already have set featured images then you can try to use http://wordpress.org/plugins/regenerate-thumbnails/ to regenerate thumbnails and see whats comes out. Since WordPress are not generating thumbnails automatically (it does but not repeatedly when changing settings and theme ) this could adjust size of your image to make them at least look closer to ones in theme demo.
It really depends on your images what will come out.
Additionally you might want to look into this blog template which has a smaller images https://colorlib.com/travelify/blog/
Or maybe adjusts theme settings for featured image size according to your image but it will take some time experimenting with image sizes. If you decide to do so you will have to tweak featured image size via
functions.php. This is value currently set for featured images.
add_image_size( 'featured', 670, 300, true );
Since you don’t have my theme enables I can’t give you more suggestions on size that could make it look/work better.