Feature image size not at set in theme

Hi

Just noticed something strange when reviewing my site scheme

Despite my features images physically being 1200px and the GP Post image Size set to full - my schema is reporting they are 840px, which does not match nay of my default image sizes

I am not sure why - and where and what i need to do to correct

I will point out that I have the following snippet to swap out the image for a smaller size on mobile (768px)

add_action( 'wp_head', function() {
    if( is_single() ) {
        $featured_img_url = get_the_post_thumbnail_url(get_the_ID(),'medium_large');
        $featured_img_url_full = get_the_post_thumbnail_url(get_the_ID(),'full');
        echo '<link rel="preload" media="(max-width: 768px)" as="image" href="'.$featured_img_url.'"/>';
        echo '<link rel="preload" media="(min-width: 769px)" as="image" href="'.$featured_img_url_full.'"/>';
        echo '<style>
        .page-hero {
            background-size: cover;
            background-position: center center;
            background-repeat: no-repeat;
        }
        @media(max-width: 768px) {
            .page-hero { background-image: url('.$featured_img_url .');}
        }
        @media(min-width: 769px) {
            .page-hero { background-image: url('.$featured_img_url_full .');}
        }
        </style>';
    }		
},1000);

I have added link to my staging site below (as my live does not yet have that schema value assigned yet)

Thanks

S