I have a PST (OL07 type) that had gotten up to 4GB. Have archived messages, and "folder size" shows about 1.7GB. I've compacted, and it appears to have successfully completed, but the file size in Explorer shows 2.6gb. I've also tried scanpst (no errors) then compressed, and still have the same differential.
Given that OL07 doesn't peak at 2GB, I wouldn't care as much, except in our Corporate network we're running Exchange03, and any time my PST file is larger than 2GB it stops updating server free/busy info.
Any help is most appreciated!