Use the PDF Optimiser / Audit Space Usage / option
This will break down the content based on %
You can then re-optimise and address there key areas
The image masks are a way Acrobat has of producing holes inside of transparent objects for other objects to shine through - so a red herring in this instance.
Hope this helps?
Content streams are pages, type 3 fonts, comments (annotations), form fields and XObjects.
If you're really concerned about the file size, one trick (although not recommended since it can cause other issues such as fonts busting out, colour transformations etc) would be to open the PDF and then PRINT to the Adobe PDF printer using say STANDARD or SMALLEST FILE SIZE ... this re-fry might will remove/re-create the content streams and reduce the overall size.