When Apple released Photos for macOS, the company chose a clever approach to reduce Photos' storage consumption, knowing that most users would be upgrading an iPhoto library. Because iPhoto retains the originally imported images without modification, an upgrade to Photos would require duplicating all of those images, plus importing any modified versions stored in the library.
I and others have explained this before at Macworld, so I won’t go into great depth, but Apple relied on hard links, a special kind of file alias that allows a file to be stored a single time on disk and have multiple pointers to that file. Those pointers act exactly as if they were the original file. You can delete all but the last hard link and the file remains on disk. (This is in contrast to aliases, which are stub files that point to another file or folder. If that is removed, the aliases break.)