Assuming the question is about
It could be also about
wp_get_attachment_link, but, although related, this analysis doesn't includes it.
Got aware of this issue answering this question: How can I view all WP generated thumbnails in Media Manager?.
Refer to it to see a working code regarding the issue on this Question.
And it lead to this WordPress forum topic.
The function wp_get_attachment_image_src( $attachment_id, $size, $icon ) returns an array containing:
 => url
 => width
 => height
 => is intermediate
 is false, the
original image data is returned.
wp_get_attachment_link rely in the function
And that's where this 4 items array is being returned.