Featured images aren't resizing

All theme demos on WP.COM use Jetpack plugin and it has Photon module that resizes image on the fly that explains the custom image url with resized info.

The reason yours images on TwentyFourteen doesn’t get cropped or resized is that you just changed theme, all the images uploaded prior won’t get cropped by theme’s own settings.

Try creating a new test post, upload image via this post and assign it as post thumbnail, see if it behaves properly in blog listing (home, archive) page. If it does good, use this plugin to regenerate image size.
https://www.ads-software.com/plugins/regenerate-thumbnails/