the reason why the library swells is that Aperture creates previews (for faster viewing, and sharing with other apps like iMovie) for each file. You can specify in the preferences how big these previews should be, either in relative size (half of original) or absolute (800x600px). Reducing the size of the preview will reduce the size of the library. I chose to set it to my main monitor size, which was much less than the default ( I believe the default is half of original).

Feel free to ping me if you have questions.

Arnt