Reducing PowerPoint file size with retina images
Try saving the screenshots to a JPG file and then inserting them into the Powerpoint. You can switch the format that Mac OS saves screenshots in to JPG by typing the following command into a terminal window:

defaults write com.apple.screencapture type jpg;killall SystemUIServer


I just tried it with identical screen shots and saving to a JPG file and then inserting into the Powerpoint made the file 80% smaller than pasting the screen shot directly into the Powerpoint. The quality was identical.



You can shrink your existing files by saving the screenshots out of Powerpoint as a JPG (right click and choose save as picture), then delete it from the slide and re-add it from the JPG you saved. 

To switch back to saving screenshots as PNG, use this command:

defaults write com.apple.screencapture type png;killall SystemUIServer