John, lets go this way: why would you want to reduce the file size?
The probable answer is that it is too heavy to share, maybe through
mail? Now lets tackle this like this:
Try using a free online presentation sharing service. After uploading
your file, you can just share the URL instead of mailing the heavy
PowerPoint file. What's more, you can also get code to embed (in Flash
format) your presentation on your blog or web site.
In your case, I recommend
http://www.authorstream.com/ (for non-
education presentations) and
http://www.wiziq.com/content/ (for
education related presentations). As your files are heavy, you
probably have multimedia content, which works well (as far as I know)
only on these two services. These services also have an associated
desktop client
http://www.authorgen.com/authorpoint-lite-free/powerpoint-to-flash-converter.aspx
(not obligatory to use), which helps organize your presentations on
the desktop, before you upload to any of these services.
I ran into this presentation a couple of days back, which provides a
clear picture on which service would work best for you:
http://tinyurl.com/ywqnjx
Let me know if this was helpful; if not, I can dig deeper.
