Powerpoint "Compress Pictures" error

Until recently my company has used Powerpoint for Mac 2011 on which you could "Reduce File Size" in a similar way to what you can with "Compress Pictures" in Office 365. This works fine until you try to use it a second time, when you get the issue of the images reducing physically in size by 1/3. I can't seem to find a way around this, even if I send it to another computer with 365. 

Could you please advise how to stop this from happening? The presentations in my company have large background images, and need multiple amends but also need to be small enough to email to colleagues throughout these amends so the feature is essential.

Here is an image of what happens to images that have been compressed more than once.