Adding Text Watermarks increases file size - how to reduce?

I am processing PDFs as part of a larger automated workflow. The workflow calls a Javascript function (via Applescript) that opens and manipulates a PDF.

My issue is when I addWatermarkFromText my file size is increasing by about 800Kb.

I am using font.Times, font.TimesI and font.HelvB (ie the built in fonts).

I also apply a preflight profile to reduce the file size, but this does nothing to the Watermark Text.

Does anyone have a method to reduce the file size using only javascript (without any user intervention)?

Thanks,

Marcus.