Turns out that Photos does pretty sophisticated library management; enough so that you're not duplicating space if you're migrating from iPhoto or Aperture. If you've migrated an existing iPhoto or Aperture photo library to Photos, Photos creates a brand new library structure but doesn't, in point of fact, duplicate your images. Instead it creates links to the original and preview versions of your images.
When you're looking at your Photos library in the Finder, it includes all of your original images and previews. But the images exist only in one location on your hard drive, even if you have multiple photo libraries.
Here's what Apple has to say for people anxious to pull the delete trigger:
After you migrate your iPhoto or Aperture library to Photos, you might feel tempted to delete your original iPhoto or Aperture library. Because the migrated library takes little additional space, you don't need to delete the original library.
So don't do it, even if you're tempted to.
You can still use your original photo library with iPhoto (version 9.6.1, as of this writing) and Aperture (version 3.6) after you've migrated it to Photos. Just bear in mind that any changes you make in iPhoto or Aperture will be resident only in those applications, not Photos. So if you make new image edits in Aperture or create a new iPhoto album, those will only appear in their respective apps, not Photos.
(Hat tip: Dave Sanderson)
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