OK, I have a Mac with a relatively small 256GB drive and right now 82GB are taken up by two caches in the system:
- ~/Library/Containers/com.apple.ScreenSaver.iLife-Slideshow-Extension/Data/Library/Caches. Stackoverflow shows that this is really a cache of JPEG files that are converted from all your HEIC files. It let's normal application run Insert Photo without having to know about HEIC. The problem is that if you have a large Photos library (like 50K files and several thousand HEIC), this thing gets huge. Right now it is 38GB. There isn't a super solution to this, but if you have an external drive, you could symlink this file to an external drive. Of course you lose that cache when you unplug
- ~/Library/Containers/com.apple.cloudpohtosd/Data/Library/Application Support/com.apple.cloudphotosd. This is another massive file that is 44GB on my machine. The problem is that if you have lots of Shared Photos, then this thing is going to be huge. So again, the only answer seems to be a symbolic link. Or turning off sharing which is kind of terrible.
The real net of this is that if you have a large Photos library or shared photos, you should make sure to get a large SSD for your MacBook for these caches.