ImageMagick vs. Gimp TIFF file conversion (file size issue)

Hello all,

I produced TIFF files from imagick6.3.5 on SuSE-10.3/64-bit.
These files are 16104166 bytes each and are in 8-bit depth.

Now, when I open such a file with Gimp, the gimp-knows-8-bit-
only-warning appears and I then have to select one of the
subimages (image 0 or image1). I selcet the first image (image 0)
and open it. Then I resave it as an 8-bit TIFF. Afterwards,
file size is only 8046586 bytes each, which is about 50 percent
of the original size.

How can I get this result with image magick (convert) for a
batch use? A convert like

   like convert file.tiff -depth 8 file-8bit.tiff

gives no reduction in file size. What does Gimp do to make those
files about 50% smaller and how can this be done with imagick?

TIA for any help,
Frank