The posts use the more tag to tell archive pages and blog overview pages where to cut off the content and show only a preview of the post.
Additionally, you can control how many posts are viewed on overview pages (like the archive) in the wordpress options. Go to Settings>Reading then "Blog pages show at most" and select the number of posts for overview type pages.
Even if you change the_content to the_excerpt in the loop-archive.php page of the theme files, it will still be putting out the content unbroken. Since the post titles are output within the elements of the post content, there isn't a way that I know of to simple remove the content without also removing the post title in this situation.