The reason your scans are larger might be because of the type of image and the cleanliness of the scan. Are your documents more 'busy' than the others you're comparing to?

Make sure that the background of the image is plain white. If there are variances of grey, then use the levels or birghtness/contrast controls in the scanner software to remove it. The more contrast, the better the compression will be.

Scan at around 200dpi if the document is intended to be printed out again in bitmap (ie. 2-colour) mode - greyscale will reduce readability and increase file size. Save the file in a non-lossy format; TIFF is best (non-print oriented formats such as BMP don't maintain size/dpi settings).

When you save the PDF, you should get options for the compression type and dpi downsampling. Set it to JPEG at a suitable quality. Turn off downsample, as your scan is already at an optimum resolution.

*edit* Just realised that you can't use JPEG for bitmaps. Select ZIP or CCITT in the monochrome bitmap images section.