Two possibilities here:
1/ you recreate your presentation. Create one slide, set the movie background you want and then right click on the slide thumbnail on the left and choose Duplicate. Create all the slides needed using this method and your presentation will be smaller.
2/ You download the latest beta (1.9.0 beta3) available from our customer “Product Updates” area: and you recreate the whole presentation: the size will be a lot smaller. But as it is a beta you may encounter problems.
I know the size is sometimes an issue in the actual version and I am working actively on this issue.