Notes is tiny until someone pastes a video into it

Text takes almost no space, which is why a notes app with ten thousand notes can be a few hundred megabytes. Attachments are a different matter, and a note is one of the easiest places to drop a scan, a screen recording or a large PDF and never think about it again.

Where it is

du -sh ~/Library/Group\ Containers/group.com.apple.notes 2>/dev/null
du -sh ~/Library/Group\ Containers/group.com.apple.notes/* 2>/dev/null | sort -h

The database and the media folder beside it are the two things there. If the media folder is the large one, the fix is finding a small number of notes rather than doing anything to the app.

Finding the heavy attachments

In the Notes app there is a view for exactly this: from the sidebar, the Attachments browser shows everything you have ever attached, grouped by kind. Sorting through Photos and Videos and Documents there is faster than any file level search, because you can see which note each belongs to.

find ~/Library/Group\ Containers/group.com.apple.notes -type f -size +50M \
  -exec ls -lh {} + 2>/dev/null | sort -k5 -h | tail -10

That prints the largest attachment files. Do not delete them there: removing a file underneath the database leaves a note referring to something that no longer exists, and with iCloud sync on, the app may restore it anyway.

The right way to remove one

1. Find the attachment in the Attachments browser.
2. Use Show in Note to see which note it belongs to.
3. Save the file elsewhere if you still want it.
4. Delete the attachment or the note itself in the app.
5. Empty the Recently Deleted folder, or the space is held for 30 days.

That last step is the one people miss. Notes keeps deleted items for a month, exactly like a second Trash, so the space does not return until that folder is emptied.

If sync is on

With iCloud sync enabled, deleting a note deletes it on every device, and the attachment counts against your iCloud storage as well as your disk. That makes clearing large attachments doubly worthwhile, and it makes accidental deletion doubly annoying, so save anything you might want before removing it.

Mail attachments and Messages attachments behave the same way and are usually larger, so they are worth checking in the same pass.