Right, well first off an OOXML file is actually a ZIP archive, and ZIP compression goes by degrees dunnit? So presumably if you just unzip it, ratchet up the compression ratio and re-zip it, you've saved a bit there.
If Paint is MS's state-of-the-art on JPG optimisation, then there's definitely a fair bit of saving you can make there too.
And are you really shocked that OOXML, a format with a 6,000-page spec, generated by an app suite notorious for the sometimes-embarrassing junk metadata it bundles into documents, has much scope for cruft-trimming? You surprise me. Either that or I missed the <irony> tags ;)