Export file not updating on-disk size

I'm working to break down this 16G PST file into 4.5G chunks for DVD backup.

1. Following the instructions from a previous post [1] I created a new Export file, opened it in Outlook, and drag-and-dropped selections of messages onto this opened Archive.
2. I ended up with one file with 6.6G. Evidently it contains only 2019 and 2023 dated messages. Ok, not ideal, but that's what backups are about.
3. My plan was to duplicate this file, then rename one 2019 and the other 2023. Which I did. Next, I deleted the 2023 dated messages from 2019 file, and all of the 2019 messages from 2023 file.
4. The result was the 2019 file was reduced in size to 0.8G, but the 2023 file has not updated it's size. I expected it to drop to ~ 5.8G, but it's not changed since this morning.
5. I've closed the archive file. I've restarted Outlook.
6. I made sure the 2019 files were deleted from the Deleted Items folder. And the folder in question, Sent Items, only lists the expected 2023 dated messages.

Didn't budge a kilobyte. How can I make this archive update it's size?

[1]: https://techcommunity.microsoft.com/discussions/outlookgeneral/reduce-outlook-mailbox-size-while-keeping-email-messages-and-attachments-and-acc/4221674