How do I significantly reduce the pdf file size?

I generally do a lot of scanning (for archiving purposes), and my files have quite a few images included. However, when I do my file size is large. Yes, I use the (Acrobat XI Pro) "Optimize File" option, the "Reduce File" size option, and the "Save As" and selection an older version of Acrobat (7.0 or later), but the file sizes are still large. 

I know there's something that I'm missing because someone sent me a large 29 page .pdf with images and it was only 356 kb.  I was very, very surprised. I asked that person and they were unaware of what caused the file to be so compressed, so I'm asking the experts.

Thank you, and additional information would be helpful.