You deleted gigabytes of mail and the PST file did not shrink by a single byte. That is normal PST behaviour - compacting is the missing step, and here it is.
A PST file is a database, and databases do not hand space back the moment you delete things. Deleted mail leaves empty pages inside the file - Outlook will reuse them for future mail, but the file on disk stays the same size until you compact it. If your goal was to step back from the size limit, deleting without compacting achieves nothing you can measure.
Compacting only reclaims space that is genuinely empty. Delete what you no longer need, then right-click Deleted Items and choose Empty Folder - until you do, every 'deleted' mail is still fully inside the PST. Use Shift + Delete for large junk you are certain about to skip the Deleted Items stop entirely. And check the quiet hoarders: Sent Items with years of attachments, and the Calendar's old attachments, are often bigger than the Inbox.
Go to File - Account Settings - Account Settings - Data Files, double-click your PST, and press Compact Now. Outlook copies the live data into a fresh, tight file and swaps it in. Two honest warnings: it takes time - a big file on a mechanical hard disk can take an hour or more - and you should not use Outlook heavily while it runs. Leave it alone until it finishes; interrupting a compact is asking for the corruption you were trying to avoid.
Compacting writes a second copy of the file before replacing the original, so your drive needs free space roughly equal to the PST's current size. If the drive itself is nearly full, compacting can stall or fail - free space first (move the backup copy you made to an external drive), then compact. This is also why 'my disk is full AND my PST is full' must be fixed in that order.
When compact finishes, check the file size in Explorer - it should now match what Outlook reports inside the file. Then keep it that way: archive by year, empty Deleted Items weekly, and compact after any big cleanup. Raising the limit only moves the wall. An oversized PST opens slower, searches slower and corrupts more easily, and the repair takes longer the bigger the file gets. The registry change is a bridge that buys you time - the real cure is archiving old mail out, compacting, and keeping every PST at a sensible size. A compacted, archived PST is the state where you never need emergency fixes at all.
Once the file is healthy and smaller, the video shows how to raise the limit safely for the mail that genuinely has to stay.
Watch Full VideoAfter every large cleanup, and a few times a year in normal use. If you rarely delete anything, compacting reclaims almost nothing - it only returns space that deleted items left behind.
You are probably looking at an Exchange or Microsoft 365 OST account rather than a PST data file, or the file is not the one selected. OST files are managed through the account's offline settings instead - and since an OST is a cache, its size follows the mailbox.
Compacting is a built-in, routine Outlook operation and is safe on a healthy file - but never run it as the first move on a file that is already erroring. Back up first, and if Outlook reports errors, repair with ScanPST before you compact.