The file you got is not unreasonably large for a 15 minute video. The problem is that video with reasonable resolution (for me small HD 720*1280) can not be all that small if some kind of quality needs to be preserved. The size like yours is really only a problem if you need to send it around as an e-mail attachment. If you upload it to Vimeo or YouTube it is fine, as it is if you use Dropbox or WeSendIt to distribute it, not to mention USB-sticks.
The ideal for non-moving slide shows would be a totally different compression algorithm, which would result in a file not bigger than pile of equally sized mid-quality JPEGs, but I am not aware of such thing, at least in widespread use. What is the size of the PowerPoint file? Many people have PP on their machines.