I also have exactly the same problem. I have a personal template file with a specific image in the background. For instance; I create a presentation with this template with a size of 2.8Mb by using PowerPoint for Mac ver.16.15. Then I try to export this
file as PDF and its size becomes 48Mb.
After that, I transfer the same PPTX file to a Windows computer with PowerPoint 2016 (O365) and export it again. Surprise, the size of the PDF file becomes just 815Kb on Windows.
I have tested so far many things and scenarios to understand the main cause of this problem. I have changed fonts for instance. But nothing has changed on Mac.
However, when I remove the background image from the template and then add it as a picture file on each slide manually, the problem disappears. But, of course, this is not an acceptable solution for me.
From my perspective, although I really like MS Office applications and have been using them happily for more than 25 years on Windows, MS Office for Mac has become a massive disappointment for me. The number of bugs and missing features on Mac version, I
have to struggle with everyday are really terrific.