When using the Merge Libraries command in PowerPhotos, it can sometimes be the case that the resulting library created by PowerPhotos is significantly smaller on disk that the original library/libraries were. There are several reasons why the new library can be smaller than the original:
- iPod photo cache. If you sync your Photos library with your iPod or iPhone using iTunes, it creates an “iPod Photo Cache” folder inside the Photos library which contains cached versions of the library’s photos, which allow iTunes to sync with your iPod/iPhone much more quickly. This cache can take up quite a bit of space (sometimes hundreds of megabytes), and if you haven’t synced the merged library with an iPod/iPhone, this cache folder will not be present there.
- Recently deleted photos. When you delete items in your Photos library, they do not get immediately removed from the hard drive. They instead get sent to the “Recently Deleted” album in Photos, and are only deleted permanently after 30 days, giving you a chance to recover them if you want. Until they either reach the 30 days or you delete them manually from the Recently Deleted album, they will still take up space on the hard drive. Those recently deleted photos are not included when merging libraries, so this can cause the merged library to take up less space on disk than the original library.
- Duplicate handling. If you have the duplicate handling option enabled when you set up the merge, this will ensure that only one copy of each photo gets imported into the merged library. If there are multiple copies of a photo in the original libraries, and the merged library only ends up with one copy, that will result in the merged library being smaller than the originals.
- Copying only original or edited version. When you edit a photo in Photos, it will keep an unedited copy of the original photo, and create a separate JPG copy that contains the edits. If you have set PowerPhotos to copy one of the two versions of the photo to the merged library (e.g. in the preferences window or when setting up a merge, then since the original library will have two versions of the photo, but the merged library has only one, this can result in the merged library being smaller than the original.
- Database/cache fragmentation. Over time, the Photos database and cache files can acquire some wasted space, from items that were deleted or changed, but for which the space was never reused. The new library is likely to have less fragmentation, and thus have somewhat smaller database and cache files. This is a relatively small effect compared to the other causes outlined above, but it can still cause a difference in size.