Optimizing Document Scanning in OSX
Rather than using the native OS X scanner, I would recommend that you download the free trial of the commercial product Hamrick VueScan. I do a lot of scanning and I rely on it. It does very well in compressing text documents to small file size.

Aside from that, you should investigate what you can do with taking scans from the OS X native scanner and then running them (as TIFF, JPG or PDF) through Automator's image manipulation utilities for further compressing the images with Apple's Quartz Filters to make them smaller.