Compressing Powerpoint files

To expand a bit on what neuroanatomist was saying, certain formats of embedded file are already compressed, including most video, many image formats (e.g., GIF, JPEG), and some audio (MP3). Many PP presentations have a lot of images, which are probably already compressed. A 39.7MB PP file which has 39, 1MB image files in compressed format only has 0.7 MB of text and formatting instructions which actually can be compressed.

You're not "missing" anything. Some forms of compression are better than others, but most importantly the content of the file will have a big impact on how effective compression is (e.g. if there are embedded video media, those will not compress much).

One possibility is to save the file as a PDF (if you just want the recipient to view the slides, not edit the file).

You could consider alternatives such as FileChute, or splitting the file before sending it (e.g. with something like Split&Concat.

Hope this helps...