File bloat when using Redaction

I use redaction often in Acrobat Pro 9 - mostly to clean up scanned monochrome (1-bit) images. All I am doing is redacting and covering the redacted area with blank (white) space. Naively, it seems to me that this would make the image files smaller.

Instead, I notice that the file size can balloon significantly when I do this. It's not unusual for the file to grow 2x, 3x, etc. When I do a space audit in the Optimizer panel, I see that most of the file size growth is in image size.

I've tried using Save As after redacting, and that doesn't write a smaller file size. Even using various monochrome compressions/downsamplings in the Optimizer still leaves me with larger file sizes than I have without redaction.

I'm lost now as to what to try next, other than exporting the images into image software and reconverting to PDF, which would be tedious. Suggestions?

Thanks,