Filter for post_thumbnail: link to small src and add data-attribute [duplicate]

Since you already know your $post_image_id, you can use wp_get_attachment_src()

$small = wp_get_attachment_src($post_image_id, 'smallest-size');
$large = wp_get_attachment_src($post_image_id, 'full');

$smallURL = $small[0];
$largeURL = $large[0];

where ‘smallest-size’ is the name of the image-size you have predefined in add_image_size()