Peter,

Acrobat is taking whatever is sent to it via the twain driver and
wrapping the result in a pdf file. The twain driver could be sending a
tiff file or a jpg file. If a jpg it will obviously be smaller. If it is
a tiff file, it could be a compressed tiff file. I don't believe that
the pdf file created with photoshop uses much in the way of compression
and I don't know if what format the raster image is saved as. If you ask
in the Photoshop forum someone might know what format the generic pdf
uses.

Mike