The usual culprit for large powerpoint file sizes are images. Usually, you put an image in a powerpoint and then scale it down to the size you need. What happens is that although it looks smaller on screen, the image is still as large as when you placed it in. (It looks smaller, but the file size is the same).
So what can you do?
If you believe the images are the culprit, there is an easy trick you can do to fix it within powerpoint. Right click with your mouse on an image in your powerpoint presentation. Then select the 'Picture' tab in the window that comes up. Press the compress button. Then choose either selected pictures or all pictures, under change resolution select print. Make sure compress pictures and delete cropped areas are checked. Then click ok.
So, what have you done? Well you have just compressed the selected (or all) pictures in your powerpoint to a resolution that should be good enough for printing (and displaying on a projector or computer screen). If you were using high resolution pictures, the change will hardly be noticeable (if at all). More likely than not, you've compressed the file by 10-50 percent. (Check by saving the file and looking at its file size.) If you need to compress more and are using small images, try doing the compress again, but select "web" under change resolution.