Yeah, it's a problem for scanning in BW and then trying to email as well.
I have found that when scanning in the BW setting doesn't actually scan in BW- or something weird is going on- the only thing that I find that helps is:
scan in your doc
then, page by page, you have to go through your doc in the Inspector, click on the "Edit Image" and finally "convert to bw".
Save and close.
that reduces file size quite a bit (by a factor of 3), but never to the <100K range...