File sizes in PowerPoint can quickly mushroom. We discussed PDF-ing and/or image compressing before to get the size of your files down.
But here are 2 additional inflators of file sizes that these techniques might not catch. The first one is logos. In order to get the sharpest logo images, you need to search for large logo images. So a typical logo page with 20 or so logos can become a huge consumer of space. But the resulting logo images are actually not that big, you can compress them further (to 96 dpi) than the other, larger images in your presentation. Make sure you uncheck the apply-to-all-images-in-this-presentation box when doing this.
Another space waster is your template. If you were guilty of Frankensteining a deck together from multiple presentations, changes are that your slide master contains duplicate copies of slide masters. Especially title pages with big images can add up. Go through your slides and the slide master to clean things up.