/**
 * Add custom image sizes attribute to enhance responsive image functionality
 * for content images
 *
 * @since Twenty Sixteen 1.0
 *
 * @param string $sizes A source size value for use in a 'sizes' attribute.
 * @param array  $size  Image size. Accepts an array of width and height
 *                      values in pixels (in that order).
 * @return string A source size value for use in a content image 'sizes' attribute.
 */
function mvi_content_image_sizes_attr( $sizes, $size ) {
    $width = $size[0];

    1500 <= $width && $sizes = '(max-width: 768px) 92vw, (max-width: 992px) 700px, (max-width: 1199px) 930px, (min-width: 1200px) 1024px, $width';
    1500 > $width && $sizes = '(max-width: 754px) 92vw, (min-width: 755px) 697px, $width';

    return $sizes;
}
add_filter( 'wp_calculate_image_sizes', 'mvi_content_image_sizes_attr', 10 , 2 );