Photos.Library folder growing despite deleting thousands of photos on Mac

This sounds excessive. I wish you had given us more information.

• How long has it been since you deleted these pictures?
• How many pictures do you have?
• How many Libraries do you have?
• Do you use iCloud Photos?
• Do you use Optimize Storage?
• Why are you still using Sequoia?
• What is your storage on this Mac? At System Settings>General> Storage, my Mac says 1.59 TB of 2 TBB used. What does yours say? Is your  free storage on your Mac less than 10%?

It can take some time for Photos to comply with deletion commands. The Photos Library isn't just a bunch of pictures, and the storage space used doesn't depend on only on how many photographs you take. Photos is a non-destructive editor, so the original picture files are never touched. The pictures we see on the screen are separate thumbnails and preview versions created, sometimes on the fly, so that we can scan through our images more quickly. Just looking at pictures can create new files! Not looking at pictures can cause the Library size to diminish. In addition, Photos scans through the Library of pictures to find details that enhance and speed up searches, for instance. The Library size is dynamic, and it keeps growing or shrinking as the need arises.

So it can take some time to do things like delete pictures, and doing those things requires storage space. If your drive is nearly full, then processes can really slow down. I've seen crazy stuff when free storage got below 15% out of 1 TB. That's why I asked about that stuff. You may want to

One thing you might want to consider is the use of iCloud's "Optimize Storage," which is pretty magic for pictures. You can use "Optimize Storage" on the Mac, on your iPhone, or on your iPad, and you can set this on any device, independent of the others. If you set a device to "Optimize Storage," then Photos may store only smaller screen sized images on the device and rely on iCloud to keep the full sized original images. This is great, since with the lower resolution images you can scan through pictures very quickly, and they look great on the screen. And, if Optimize is chosen, and you want to edit or crop a picture, Photos will reach out to iCloud to get the full sized Original for you to work with. It's the same for editing or printing or anything that demands the full picture. Your optimized Library may take up less than 20% of the space of a fully downloaded Library. On my iPhone, Photos takes up way less than 10% of the space it uses on my Mac, because I don't do editing on my phone, so there is no need to download the originals at all.

I don't use Optimize on my Mac, because then backups are more complicated. I don't keep all my pictures in my System Library-- all but the favorites are kept in other Libraries that can be on external drives. If the Drive is formatted in APFS, then those external Libraries can be opened by Photos.

How do you back up your pictures? An Optimized Library can be backed up to an external drive like this:

Backup iCloud Photos with an Optimized Mac - Apple Community

sfreund22 wrote: How long has it been since you deleted these pictures? 1 day

It may take more time than that.

How many pictures do you have? 125,00 pictures

You should think about using multiple Libraries and only synchronizing a smaller set with iCloud. You can keep the other Libraries on external drives. I have a Favorites Library that is only 17000 pictures-- those are the ones I most want to look at or show to others on my iPad or iPhone. I don't need to have all the others synchronized. The smaller Library fits on my MacBook, so I just backup the Mac with Time Machine. No worries.

Do you use Optimize Storage? I just did. But this is confusing. If I want to not just back up but download my original photos and permanently remove them to external drives, don't I have to turn Optimize Off?

See the link I gave earlier:

Backup iCloud Photos with an Optimized Mac - Apple Community

Okay, it looks like my numbers have just updated. My Mac shows 565.28 GB used out of 2 TB. And now EVERYTHING (photos and other files) is optimized.

Great! As I had said, it can take some time.

What do I do when I want to download and physically remove photos (or documents) from my hard drive.

iCloud Photos is completely separate from iCloud Drive, so Optimized pictures and documents behave differently. iCloud Drive optimizes by keeping pointers to files at iCloud.com. Photos optimizes by keeping lower resolution images locally and using pointers to higher resolution originals at iCloud.com. Low resolution documents makes no sense.

Both systems work transparently. If you want to delete a picture on your Mac, just do it. That deletion instruction will be copied iCloud Photos and then to all other connected devices. You don't do any "downloading." If an original picture file is required for an operation like editing, then it is retrieved automatically.

For iCloud Drive, start here:

Understanding iCloud Drive - Apple Community

2. When I download my photos to archive them, should I choose the regular choice or the Originals.

You archive the Library rather than individual pictures. The Library has way more information than the Originals and the Edited versions combined.

3. Following from this, can I just turn Optimize on and off at will, when I need to work on or move the original files, whether they be photos or files?

If you turn off Optimize, the Originals aren't immediately loaded on to the Mac. You end up with an odd mixture, and there's no indication of when the Originals are fully complete. It's better to just start over with an empty Library and fill it. Again, you can use the process outlined in the Backup link above. If there are specifics from that link that need expanding, let me know.