Our engineers have analyzed the content of OST (source) and PST (destination) files. The analysis revealed the types of data that are not carried over to the PST file after OST file conversion:
In other words, when you remove or modify a message, contact, reminder or meeting, the object is only marked as removed, but is not removed physically and remains in the OST file. The new version of the object is written to a newly allocated space, but the old version remains in the OST file as well.
If Outlook is in active use, the size of the OST file can grow quite significantly, while the actual size of data in the file remains the same.
In order to optimize disk space consumption and remove old elements, you need to configure the AutoArchive feature in Microsoft Outlook.