Omni DiskSweeper (free) will show you the storage requirements for each of the Finder folders in your Aperture Library file package. I think that will allow you to answer your question based on your data.
It's a sweet little utility. And in use should be straightforward. If it's not, post back.
Message was edited by: Kirby Krieger -- clarity.
Is 16 GB a normal size for a referenced library? Are the previews using this much space on the drive? I have the preference for previews at 2560 x 2560.
A quick calculation:
Your previews will have roughly 5 megapixes with these settings (2560 x 2560 x 3/4), and depending on the complexity of your images (the amount of detail) the size of the jpegs should vary between 1 and 2 MB.
So with 20000 images in your library, your high quality previews will need ~15 GB, also your library may contain an iPod Photo Cache, if you are syncing with iTunes, plus thumbnails.