If you have a child theme of some of the more recent WordPress Themes (like Twenty Fourteen for example) you may notice that the images your set as your “Featured Image” are automatically resizing. WordPress has some pretty handy features built in that will automatically “optimize” your site for you. By taking the images you uploaded and resizing them not only will they fit better into your template but they will load more quickly for end users.
The following code is responsible for this in the Twenty Fourteen Theme (and any child theme as well):
// Enable support for Post Thumbnails, and declare two sizes. add_theme_support( 'post-thumbnails' ); set_post_thumbnail_size( 672, 372, true ); add_image_size( 'twentyfourteen-full-width', 1038, 576, true );
Just one more thing that WordPress does to makes your life easier and end users happier.