Fetching full size featured image to insert in RSS Feed

Question

I am trying to fetch a full size Featured Image (or first image) of the post to appear within my RSS feed. See here

I am using the following code

function firstImg( $post ) {
    $matches = array();
    $output = preg_match_all( '/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches );
    if ( isset( $matches[1][0] ) ) {
        $first_img = $matches[1][0];
    }

    if ( empty( $first_img ) ) {
        return '';
    }
    return $first_img;
}
add_filter('the_excerpt_rss', 'firstImg');

However, this is only fetching a 200x200 thumbnail size of the image. I want to use the full size which is 600px width.

What's missing in the code?

Thanks