People often ask me how much smaller Open XML documents are than corresponding Office binary documents. It's a hard question to answer with any precision, because the difference in size is so dependent on the document content. For example, a long simple text document will compress by orders of magnitude, a typical document with graphics and other types of content compresses somewhat less, and you can even create documents that are a bit larger in Open XML format than they are in the binary formats.
Gray Knowlton's post on "File size reduction for Open XML" covers some of the issues involved, and explains what you can expect in general terms. Gray is a group product manager in Office, and I'm on his team. We have a lot going on around Open XML these days, and this post is the first in a 3-part series he'll be doing on file size reduction, document "sanitization" and improvements in document format security. You can subscribe to Gray's blog right here, and you can expect he'll have some interesting things to say about Open XML going forward.