Ok, I found the solution.
When I select an Adobe PDF Preset (High Quality Print, or Smallest Filesize), the Dropdown menu for "Standard" jumps to "None". I have to then manually select "PDF/X-1a:2001", and only then will my exported PDFs have a normal size.
I find it strange that the PDF presets don't have a sensible standard selected automatically. No one expects a 700MB PDF after selecting the default setting "Smallest File Size".
Ok, I found the solution.
When I select an Adobe PDF Preset (High Quality Print, or Smallest Filesize), the Dropdown menu for "Standard" jumps to "None". I have to then manually select "PDF/X-1a:2001", and only then will my exported PDFs have a normal size.
I find it strange that the PDF presets don't have a sensible standard selected automatically. No one expects a 700MB PDF after selecting the default setting "Smallest File Size".