I assure you that for a 15 minute Captivate presentation 8 megabytes is by no means large. If you intend having smaller file sizes, make your project files smaller. Maybe 5-7 minutes max.
I have not seen anything you've said so far that would explain your other issues. If playback is very slow to load then your end user network bandwidth might be severely restricted.
Are you publishing this as HTML5, or HTM/SWF?
