The reason is simple, the files are already compressed. Office 2007 documents(.docx,.xlsx,.pptx etc) follows OpenXML formats.
Usually we can save lot of disc space if we Zip (or any compression method) if the documents created with previous version of Microsoft Office(.doc..xls etc.). we do this especially before attaching to E-Mails.
But it’s not really necessary with the new format as compression is one of the main features of OpenXML format. Even if you compress Office 2007 documents, you won’t be able to see any good change in the size of the files.
Microsoft Office Website has listed the benefits of OpenXML format
Compact files – Files are automatically compressed and can be up to 75 percent smaller in some cases. The Open XML Formats uses zip-compression technology to store documents, offering potential cost savings because it reduces the disk space required to store files, and decreases the bandwidth needed to send files via e-mail, over networks, and across the Internet. When you open a file, it is automatically unzipped. When you save a file, it is automatically zipped again. You do not have to install any special zip programs to open and close files in the 2007 Office release.
Inspired by blog post: Don’t Worry About Zipping Office 2007 Documents
