SP1 is a rolled up bunch of the updates released over the last year. Thats why the download is 434mb in size instead of seeing each added to the list while at the update site.
Once you have it on go there and select the custom not express option and review the update history option in the left column seen there. That will open up a page displaying a long list of security updates as well as things for Windows Defender, WMP 11, etc. you would accumilate over a period of time plus some new ones just added in.