It could be that when you inserted the image in the content area the image resized to 600px to fit the TinyMCE Window?
What happens when you insert the image if you enter 642px in the width box? Does it still resize it
Also as a note to stop SS reducing quality in images you can set the global quality in your mysite/_config.php file by adding this line
// Make all images nice and fancy
GD::set_default_quality(100);
By default its about 75 I think so this will increase load times and CPU work that needs to be done therefore it will use more PHP Memory as well so if you are on a slow shared host I wouldnt recommend it! but it will give you crisper images