When you migrate a photo library from iPhoto or Aperture, the Photos app creates a new library structure, but, according to Apple, doesn't duplicate your images. Instead, Photos saves disk space by creating links to the original and preview versions of your images.
When Finder reports the file size of your Photos library, it includes all your originals and previews. It may look like your remaining iPhoto or Aperture library is taking up twice the space on your hard drive, but it isn't—your images exist only in one location, even though you may have more than one photo library.
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.
You can still use your original photo library with iPhoto 9.6.1 and Aperture 3.6 after you've migrated it to Photos. However, you should note that any changes you make in iPhoto or Aperture after you migrate to Photos, like image edits or album changes, will be visible only in iPhoto or Aperture and not in Photos.