BcScan is hyper minimal PDF scanning program for OSX I just wrote because I couldn't find anything else that I liked.
BcScan is bare bones, like a copy machine. You put the App in your dock and click the Icon. It starts black-and-white scanning immediately and opens the scan in preview. Leave preview open to add more pages, or close preview to start a new PDF. The PDFs go on the desktop and the file size per page is < 100k.
BcScan requires you to install the SANE drivers, which, unfortunatelly, come as four different DMGs. I'm sorry, that's only way to get the SANE drivers for mac, but at least I made the version selection automatic, so you can just click-open-install each file and will get the right version.
As a reward for your installing efforts, you get support for older scanners and scanning seems to be a lot speedier as well (YMMV), and you won't need to spend 50 bucks or waste time doing preview scans or converting colors and formats.