I don't know what is going on but it looks like others are hitting this "bug". With "don't copy" checked, my 400GB photo database is generating a 250GB (see Apple support comment on discussions.apple.com, macrumors and on reddit) which is consuming essentially all of my hard disk. There's some debate as to whether these are hard links or not but it appears that the "don't copy" feature is basically useless, if you want to use Photos, you have to nearly double your disk requirements.
Basically, it is all or nothing, you need to commit to Photos (unlike iPhoto) and send your photos into it. For me, this is a real problem as I've had iPhoto crash quite a few times losing its entire database (I don't think it likes 180K photos :-), so on to other solutions. Probably will try Lightroom, but keep my file system storage method so I don't have to commit to any one database format or company and use standalone tools that manage it.
Here are some more of the details:
Some of the other reports are:
- 113GB library generates 32GB Photos library
- 250GB of jpegs generates 110GB Photos library
- 90GB jpegs generates 55GB Photos library
This looks like something that is because of the faces feature, in trolling through model resources, I see things that look like thumbnails, but quite often I see a 2MB photo that is copied there without a face. Looks like it is getting confused and copying whole images over that it doesn't recognize.
Makes Photos kind of useless on machines without lots of disk space!