Increase in Photo library size after sync with iPad

I have not used the manual syncing using the Finder for a long time, so I cannot test, if it is behaving still as it used to be on older system versions.

When you sync photos or videos to an iPad or iPhone, the items will be synced in an optimized resolution to the device, for viewing on the smaller display and to save storage on the device. Because it takes a lot of time to render the smaller versions, Photos is caching them. It will create a folder "iPod Photo Cache" (iPhone or iPd Photo Cache) in the Photos Library. You can check, if such a folder still exists on your current system version by ctrl-clicking the library and using the command "Show Package Contents". We can remove this folder to save storage, but then it will be recreated when you sync again and the next sync will take much longer.

The library size can also increase, when you edit the synced photos on your iPad and save the edited version on the iPad. Then My Photo Stream will sync a smaller version back to your Mac and you will have a duplicate with a lesser resolution on your Mac.

The best way to avaoid the loss of storage because of the cached synced photos would be to be very frugal with the number of photos you are syncing to the iPad. Sync only selected items, your absolute favorites, and not all photos.

First, before your mess with your Library contents, make a backup-- drag the Library to an external hard drive that is formatted in either APFS format or Mac OS Extended (Journaled) format, and that hasn't had Time Machine on it since it was formatted. It might even be best to do any experimenting on the external version, since it sounds like you don't have much free space on your internal drive.

Once you have a full backup you might try rebuilding your Library by closing Photos and option-command-clicking on the Photos icon. I tried this on a small Library that I've been editing, and it got smaller-- only 2%, but you might get better results, and it's easy.

It's hard working with a crowed drive-- you spend so much time thinking about what you can get rid of, and worrying if some new will wipe out your space! You might want to consider operating your Photos Library from an external drive. Solid State Drives can be very fast and quite portable.

If I synch my [now bloated] Photo Library with my iPad again, only this time synching only certain photo albums, will that free up HD space?

If the increased size is caused by a photo sync cache, a new syncing with less photos selected should decrease the size of the cache.

Apple has removed the documentation about the photo cache folders from the knowledge base. The documents are still saved by the wayback machine: https://web.archive.org/web/20151222081002/https://support.apple.com/en-us/HT203156

You're welcome😊. Good Luck!