There are two different ways you can add these graphics to your PowerPoint presentation.
- Choose Insert > Picture > From File (or Clip Art)
- Paste the picture or clip art into the slide
When you paste a graphic from another application into a slide, PowerPoint creates an image that cannot be compressed by PowerPoint, so the end result becomes a larger PowerPoint file.
When you use the Insert > Picture > From File method, PowerPoint can compress this image so the file size will become smaller than if you had simply pasted it in from another application.
What is really interesting though, is that once you have inserted the image using the Insert > Picture method, you can then copy the image and paste it to another slide, or even another presentation, without greatly increasing the file size. Go figure.