Same thing happens in IrfanView.
I just scanned some notes, and because it was a rather big number of pages, I decided to use express scanning and to crop them afterwards, in Paint.net.
The problem is - paint.net increases the filesize of these grayscale scans significantly, despite the fact, that the images, after cropping, are smaller. Here's an example:
http://i48.tinypic.com/9k2f7p.jpg
This image is 600 KB, straight from the scanner. To keep the file size at 600 KB, after cropping, I have to lower the quality down to 67%. The decrease in quality isn't that noticeable, as these are no complex images (they're grayscale), but it's still worrying that I have to dumb it down to keep it small.
Any thoughts or tips on this?