I've been using this to make thumbnails of jpeg files:

for img in *.JPG; do
echo thumbnailing $img
convert -geometry 105x70 $img $img
done

Files from the K10D result in thumbnails of about 57 kB and those from
the GR of about 46 kB, with little variation. Is there a simple
explanation as to why?

The K10D is 10 megapixels and the GR 16, but I don't see why that is
relevant here, but maybe I am missing something.