I use an Acrobat plugin,
Quite A Box Of Tricks, to resize images in PDFs. It works with Acrobat's batch processes, so it can do an entire folder of PDFs automatically.
However, it is an Acrobat plugin... you need the full version of Acrobat to make it work, and Acrobat Pro to use batch processes. And it's not cheap:
UK pounds 125.00 plus 18.75 VAT at 15% (tax)
US dollars 205.00
Euros 199.00 plus 29.85 VAT at 15% (tax)