Can you share details of the configuration of your setup ?
I found a solution.
The compatibility setting in most of of my PDF presets, including most of the "canned" presents, is set to Acrobat 4 (PDF 1.3) or Acrobat 5 (PDF 1.4). This works fine for files exported from InDesign CC 2015.3, but results in much larger exports from the 2015.4 release. I changed this setting to Acrobat 6 (PDF 1.5), and now exports from the 2015.4 release are the size I expect.
So glad I don't have to roll back. I'm keeping 2015.3 on my laptop for now, just in case, but now that this problem is solved I'll keep trying out the new release on my Mac Mini.
Is it possible that you can share the file with us with which you were seeing this issue? I have inboxed you the email id.