[Macworld has a great tip](http://www.macworld.com/article/1168311/shrink_preview_files_without_ruining_image_quality.html) on how to shrink Preview files without ruining image quality. Essentially, it entails navigating to:
and then copying the file `Reduce File Size.qfilter` to some place where you can edit it. No fear: it’s an XML file, which means you can produce multiple versions for different effects, making sure you give the versions different names so you can move them back into the `Filters` directory when you are done. (You will need to be able to authenticate to do so, just as you had to make a copy of the file elsewhere in order to work with it: Mac OS X does not want you editing its innards live.)
The parameters you are going to adjust are: Compression Quality and ImageSizeMax. The article has some good suggested values, which are a good place to start as you tweak things for your own benefit.