copying and pasting images from PowerPoint to Word - why is the file size huge?

I create instructional materials for training. The documents are Word documents, with images of slides pasted in. Just lately, I've discovered that when I copy and paste images into a word document, the file size gets huge - much too big to send back to my clients through email. This shouldn't be the case. I've tried clicking reduce file size, compressing all images, and clearing all track changes, and that reduces file size slightly, but not enough. Is there something I'm missing in Word or PowerPoint that can reduce these images, or is there perhaps a setting on my Mac I need to change to make pasted image file sizes smaller? I know when I had a Windows laptop, I had to adult the image default settings from raster to vector, and that reduced image file size greatly. I'm asking this same question in an Microsoft community, too, and really need to find a solution. Any help would be appreciated.