Library size discrepancies

When using iPhoto Library Manager's "Rebuild Library" or "Merge Libraries" commands, it can sometimes be the case that the resulting library created by iPhoto Library Manager 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:

• iMovie thumbnails. If you have any movie files stored in your iPhoto library, and you also use iMovie on that machine, it will create small "thumbnail" versions of each movie in your iPhoto library. It uses these thumbnail movies when you're browsing through your iPhoto library's movies in the iMovie browser, to allow you to look and scrub through the movies quickly. These thumbnail movies are stored inside the iPhoto library itself, alongside the original movies. While usually smaller than the original movies, they can still take up a significant amount of space, many megabytes a piece. If you have a large number of movies in your library, this can multiply quickly to take up large amounts of space, even multilpe gigabytes in some cases. The rebuilt/merged copy of the library, however, has not yet had these thumbnails generated by iMovie, so that can account for a large difference in the size of the new library.
• iPod photo cache. When you sync your iPhoto library with your iPod or iPhone, iTunes creates an "iPod Photo Cache" folder inside the iPhoto 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 rebuild/merged library with an iPod/iPhone, this cache folder will not be present there.
• Photos in the trash. If your original library has a bunch of photos in the iPhoto trash, those photos are not included when rebuilding or merging libraries. The actual photos still exist on your hard drive though, and continue to take up disk space until you empty the iPhoto trash in the original library.
• The "Don't import duplicates" option. Similarly, if you had this option checked in iPLM's preferences or your merge setup, and any duplicates were encountered during the rebuild/merge, the additional copies of any duplicates will not have been imported into the new library.
• Database/cache fragmentation. Over time, iPhoto's 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.