I have file I created in Indesign that has a lot of grayscale images. It's true grayscale and I made them grayscale in Photoshop. For whatever reason, this pdf is ballooning up to 18 meg and it's only 10 pages. It didn't start happening until I converted the images to grayscale.
What is your color management policy for CMYK?
What are the PDF export settings? You export?
The export of grayscale images is handled differently than export for color images. Check Export>Compression and look at your settings for Grayscale