Hi,
The featured image must be bigger than the minimum resolution of thumbnail for proper cropping to take place. If the images are big already and still thumbnails are of different sizes then you have run this plugin once https://wordpress.org/plugins/regenerate-thumbnails/
There is one more option in case it suits your site.
line 54 of functions.php is
add_image_size('excerpt-thumbnail', 200, 140, true); // Sets Index Page Thumbnails
You can try to stop cropping from center by setting true to false.
add_image_size('excerpt-thumbnail', 200, 140, false); // Sets Index Page Thumbnails
For more usage and examples please refer:
After every change, you will have to run the regenerate thumbnails plugin.
Now that you know all options, I do think that the current image settings of the theme are the best suited for most sites.
— Also check https://demo.themonic.com/io-pro/2/ on your device and do email the site link to [email protected] so I can check for issues.