The first one uses single.php with the class narrow and the second one uses index.php with the class bigger.
This could also be a setting in your theme options in the dashboard.
Otherwise check these files out in your themes folder.
Ok I changed the width for #narrow
Thanks a lot
But we can see a problem with the meta.
At the footer, the band go out of my page.
What could be this problem?
Ok Arno,
add this to the #meta:
margin-left:14px;
margin-left:14px;
and: Get Firefox and then the free Firefox addon Firebug. You can then click anywhere in your site and see what piece of the css influences that piece of your site and you can even make changes but not save them, so it is safe to practice.