this post is intended as a help to those who may have encountered this problem.
I was using the great "Monochrome Pro" theme from GraphPaperPress but whenever I then loaded a page/post that had a featured image set, the featured image did not resize to fit the header area properly (as it should)...instead it displayed in it's large size!
If you have had this problem, the solution is the following:
In the theme's header.php file, locate the line:
echo get_the_post_thumbnail( $post->ID, '950x400' );
and replace it with the following:
echo get_the_post_thumbnail( $post->ID, array (950, 400) );
..and that's all!
Hope that helps.