If you have bitmaps in your presentation, convert them to Picture (Enhanced
Metafile). This is easy to do and sometimes actually improves the graphics as
well as shrinking the file size.
On each slide:
1. Select graphic.
2. [Edit]-[Cut].
3. [Edit]-[Past Special] - paste as Picture (Enhanced Metafile)
After saving and exiting from PowerPoint, you may well find your file size
is smaller.
Enhanced metafiles contain a combination for vector and bitmap graphics and
are often the best compromise considering clarity and space.