If the image size does not exist, usually because the original image is smaller then the defined image field then the full size image is returned.
When using wp_get_attachment_image_src() the return arrays 4th element will be true if the image at that size has been resized.
I do not know if this information about the image is returned in the image size details.
Find out
$theImage = get_sub_field('hero_image');
echo '<pre>'; print_r($theImage); echo '</pre>';