Graphene – Image Width Limit

Hi there,
I’m using Graphene 1.1.2 with a child theme. In the original theme’s functions.php is the line

if (!isset( $content_width ) )
	$content_width = 580;

In my child-theme functions.php is in the very beginning
$content_width = 651;
Now, when trying to insert an image which is for example 1000px wide, the “Big” image still is 580px wide. ( Image )
If I replace the 651 with something smaller than 580 it works and my “Big” images have a max width of this value.
So there has to be something that limits the image width to 580.
I’m not sure whether this is related to Graphene ( my theme ) or if this is a general issue.
I hope you can help me here.

Yours,
Philipp