The PST for Internet Calendars does not automatically compact.
Every sync is an entire, new copy saved into the PST.
I found my file is 42GB (I have about 10 calendars).

To see if you have the issue, check
C:\Users\(username)\AppData\Local\Microsoft\Outlook

If you have nothing gargantuan, then all’s well.

If your Internet Calendar Subscriptions.pst is huge:
1) you can compact it for now:
* Outlook
* File
* Account Settings
* Data Files
* Click on Internet Calendar Subs
* Click on Settings
* Compact now
* It takes 4-5 seconds per meg on SSD…

2) You can change how often this downloads.
* Outlook
* File
* Options
* Advanced
* Scroll down to Send and Receive
* Click Send/Receive button
* Edit your send/receive groups to sync Internet Calendars less frequently

3) You can set Outlook to automatically compact on close.
The drawback is that if you left it running a long time,
close might take hours. It’s also a registry change.
Because of this, I’m not listing steps for it,
but Uncle Google can help.