PowerPoint files can be very large, especially if you use high resolution images, or even videos. In the near future, all of us will have moved to a cloud-based storage solution, where we no longer email actual documents to each other, just a link. In the mean time, we need to try to keep our PowerPoint files below 10MB, the email architecture limit that was invented in the 1990s.
In the format pictures menu of PowerPoint is an option to compress images to save space, I usually go down to 150DPI. After compressing, always check what happened to the images in your presentation, the compression tool can do funny things (ruffle transparency borders, undo re-colouring, etc.)
If your files are still big after compression, or even when you do not use any images at all, have a look in the slide master (view, slide master view). There might be templates for title pages and/or separators there with full-size images. These get saved with the entire presentation. You can delete the template slides if you do not need them.