PDF exported from Publisher is huge

[KarinC]

I see this come up now and then but I can't figure out what is going on. 

I had exported my illustrated book last summer as a PDF/x-1a:2003 at 300 dpi with jpg compression at 85%. I got a file that was about 250 MB with about 50 large jpgs, 61 pages total.

Since then, I have added about 6 or 8 more pages with jpgs and exported it as a PDF1.7 at 144 DPI (for online review) and get a PDF file size of 386MB. I reduced the DPI to 96 and get a PDF of 386MB. I unchecked 'all layers' and just about everything else. Why won't the file size reduce with a lower resolution? I will never be able to submit this if I try 300 DPI since it can't be over 650MB.

I'm using linked images.

I don't think I'm going to survive to see this book done. I think I've solved one problem only to get a new one.

Edit: I'm using Publisher 2.4.1

[KarinC]

I finally got a smaller file size (13MB)  if I selected downsample images at a lower number, 144 instead of 300. Maybe if I play around with resampling I can get a better quality.