Why is my .pdf so much larger in file size than the InDesign file I created it from?

I have a 17MB InDesign file. Not a huge file size for a 40 page booklet. When I convert it to a pdf it blows out to 70MB! Usually the file size is reduced when saved to a pdf. I can't figure this one out. I'm saving pdfs as I usually do and have never had trouble before. For proofing purposes I'm able to lower the file size in Acrobat, but I can't do this for the final print file as I need high res images. Any ideas as to why the file size is so large when I convert it?

Cheers.