wp_get_attachment_image_src always returns full-sized image

https://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src#Parameters leads me to believe wp_get_attachment_image_src( $thumb_id, array(100, 100) )[0] would return a 100×100 thumbnail, but it always just returns the full-sized image.

Any help would be greatly appreciated.

I think this code will help you:

$src = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full', false, '' );
echo '<img src="'.$src[0].'">';