A common issue with Acrobat: file size.

I've been scanning in documents (B&W, letter sized). I then import them into Acrobat. These files will go on my company's web site. Unfortunately I can't get a proper size/quality ratio. I have example files from another company that they emailed to us. For example, I have a very nice quality, crisp B&W, 10 page pdf from them that is only 500KB. That's great, and I'd like to achieve that. However, the best I can do is a poor quality 6 page document that's about 400KB. So basically, I'm doing about 1/4 of the result I want. Unfortunately, I don't know Acrobat well enough.

I've been using Irfanview to batch convert the images with countless different settings. I've shrunk the sizes of the images to a variety of sizes, I've tried changing them from 2 to 256 colors, and I've tried saving them as BMP's, JPEG's, and TIFF's. Suprisingly, the BMP's worked best, probably because that's what they were originally, but the JPEG's were the largest by an extremely wide margin.

What am I doing wrong here?