As it turns out, I have another computer (that I recently purchased) that syncs with the same iCloud account. Photos only take up 28GB. It is also set to "optimized." Did the older computer simply download a lot more photos than the newer one?
I think the local cache of images Photos has saved may have just grown. I think the rules for what gets purged are very much in keeping with other iCloud services (i.e. it does it itself when it decides it's necessary and you have no way to manually "evict" things that it's cached).
If you're 100% confident everything on that Mac is uploaded to iCloud, you could just delete the Photos library on that machine and let it start over. The cache would be much much smaller, at least to start with. A good way to verify what's uploaded is to go through the web interface at iCloud.com and see what you see there.
I would add something important here:
make sure that somewhere you have everything backed up. If you own 50,000 photos you intend on keeping, and every Mac you use is set to Optimize, that means there are lots and lots of photos that are ONLY stored on Apple's servers. That's not great! If anything happened to their server or things got corrupted or whatever, your stuff is gone.
What I do is I have my iMac set with Optimize turned
off, so that every photo and video in Photos is saved locally. Then I make sure that .photoslibrary file is backed up to another hard drive (two actually, with one being kept offsite). My laptop is set to Optimized, which is fine because the iMac is doing all the heavy lifting.