What is being stored in an ost-file also depends on whether you are using an Exchange or IMAP account. When you use Outlook 2013, 2016, 2019 or Office 365, the ost-file can also actually be smaller than your mailbox.
Of course, in most cases, your own mailbox does make up the large part of the ost-file but there is much more stored in there which can be quite big or even bigger as well! Looking at the size of your ost-file isn’t really representative for the size of your mailbox. What accounts for this difference and how can I make my ost-file smaller? However, when I look at my ost-file, I see that it is over 3GB in size. We have a maximum mailbox size of 2GB and I’m currently using 700MB of it.
