I've noticed that you can often substantially reduce the size of PowerPoint files simply by saving the same file to a new filename. I just did this for a lecture on social networking theory, and it went from 7 megs to 3.5 megs.
I'm trying to guess why PowerPoint does this, and not coming up with any good ideas. It can't be for undo, since PowerPoint eliminates your undo queue whenever you normally save. It might be for faster saves, though I never notice any difference between saving normally and saving to a new file.
At any rate, one thing that would be really nice would be something that did this automatically before emailing it out or posting it on your web site, just imagine the savings!