Could you please share the link to your website?
This can happen sometimes when the images have different sizes and the theme doesn’t know how to resize them or use a specific size for featured images.
Ok, even in the theme example they have different featured images with different sizes. This means the theme is not cropping the image to fit a specific area.
In this case your best choice would be to manually change or crop the images to always have a specific size in pixels.
You can find more information about cropping images inside WordPress (and editing in general) here: http://www.wpbeginner.com/beginners-guide/how-to-crop-rotate-scale-and-flip-images-in-wordpress/
Please let me know if you need further help
