Huge file when exporting iPhoto slideshow
Your best option I think is to reduce the size of your images. This can be done with Preview but is doesn't run in batch mode. There are third party apps like Pixillion that will change your image to a different format or just reduce the quality slightly resulting in a much smaller image. Pixillion has a trail period so you can play with it and see if it works for you. It also appears to have a batch mode option. I saw a 50% reduction by degrading quality by a slight amount. You should play with this to see when you start noticing degradation in the image. See Pixillion panes below.



After pressing the "Compression Settings" button you get the pane shown below.



Another option is the App Squash. I don't know how this pwrforms but it sounds like it may do what you want. Developers do not offer a free trail period so its $15 and cross your fingers it helps you.

The last option is the one I mentioned earlier. In preview you can go to the  tools menu and click on Adjust size as shown below.



When you click on adjust size you get a pane that looks like the image shown below. As you can see you can adjust the image size or the resolution. You can play with these parameters to find the best values.