Link to larger thumbnail in custom query (with custom post-types)

In your code you are linking to an image using this code:

wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), "medium" );

So what you are doing is showing the medium sized image as well as linking to the medium image.

What you need to do is link to the full sized image:

wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), "full" );

Also wp_get_attachment_image_src returns the following:

array
(
[0] => url
[1] => width
[2] => height
)

So this:

<a href="https://wordpress.stackexchange.com/questions/25718/<?php $thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ),"medium" ); ?>"><?php the_post_thumbnail('medium'); ?></a>

needs to become:

<?php
$thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), "full" );
?>
<a href="https://wordpress.stackexchange.com/questions/25718/<?php echo $thumbnail_src[0]; ?>"><?php the_post_thumbnail('medium'); ?></a>