Right now aperture is deleting 108K+ previews that were taking up 350 GB of space, without any really use.
What actually happens? It’s quite simple. Whenever you first import, and then make any adjustment (crop, exposure, etc.) to a photo, Aperture renders a new JPEG of that photo with the adjustment applied. It renders this from the RAW file, applying the adjustment you’ve done, and generates the JPEG. This is what allows you to drag a picture from Aperture onto the desktop, into iChat, or anywhere else. Aperture will also update the XML database, which allows other applications like iMovie, Pages, iPod syncing, and even the Finder to access your pictures through the Media Browser. In fact any application can be written to access the Media Browser, allowing your Aperture library to be accessible by any app in the OS. It’s very cool.
Those JPEG files are what are taking up all that space. While a JPEG is nowhere near as big as the original RAW file, when you have tens of thousands of photos, it adds up. The XML is insignificant in size, but does contribute to how long it takes to quit Aperture—that “Updating information for sharing previews” message on quit is Aperture update the XML file.
I found the information to reduce my Aperture library size down to 50GB here: