The only additional size that is added by frontier theme is the 200×120 which you can remove from functions.php
The line is:
add_image_size( 'thumb-200x120', 200, 120, true );
But there are more image sizes that are created besides the original upload. Those are created by WordPress core and is not controlled by the theme. So once you upload an image, those sizes will be created regardless of what theme you’re using and I really don’t know whether it’s possible to turn those off. Maybe there’s a plugin somewhere that could remove those for you.