Just sounds like it's being scanned in an uncompressed ("raw") format. Open it up in preview, do a Save As, and change to some lossy format like JPG.
I ended up doing a B/W scan at a lower resolution (my lawyer said it would be fine) and the document shipped okay - and before your post. However, I did run a test using your solution and, yes, that colour JPG would have shipped just fine. Thanks for the tip.
