Enormous size of IMAP .pst file

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Outlook version
Outlook 2010 64 bit
Email Account
I've been using MS Outlook for many years and probably this question will be on a level of a caveman but
can anyone explain me what exactly is stored inside .pst file which is being created when I set up email account in IMAP?
You can use a program called MFCMAPI to look at the hidden things, just don't delete anything. The IMAP pst in Outlook 2010 has mail, some hidden messages for rules, views and other things. The message overhead can be as much as 25% (Outlook msg format compared to the IMAP server's mbox format), so it will be larger than the mailbox on the server but how much is specific to your mail. Larger messages and messages with attachments grow more in the conversion than small messages. I would expect about 10% larger.
So just to clarify; it contains only emails and it can't contain data like contacts, tasks or notes? If I delete it will everything download again from a server?
Another thing I don't understand is if my IMAP .pst file has like 10GB and I delete it, why after it's recreated it's much smaller than before, like 2GB. What happens with the rest 8GB?
IMAP is email only, so yes, in Outlook 2010 you'll only have email in it. Outlook 2013's IMAP accounts can have calendar and contacts in local "This Computer Only" folders only. It's a poor decision in my mind, too easy to lose them and they can't be recovered.

I have no idea what happened to the 8 GB but one can assume the data file was corrupt or Outlook went nuts, and you have in the neighborhood of 2 GB actually on the server. I've never had it happen to me personally but have heard tell of as much as 50 GB data files when the server had much less.

One thing that can cause a large pst is RSS feeds. I don't know if you can deliver RSS feeds to an IMAP data file in Outlook 2010.
I've noticed that in Outlook 2013 there is no designated .pst file other data than emails and IMAP files are now called .ost. If I have five email accounts configured in Outlook 2103, in which .ost file my contacts and the rest will be stored? I'm guessing in the one that is chosen as a default one but what if I change a default email address? Will all contacts will be transferred to a different .ost file or since then they will be saving in that new default .ost file?

I checked the size of Gmail from the Gmail app and it's actually 32GB. Is there something I can do about it (like maybe switch to POP and keep it organized on the computer)?
In Outlook 2010, shall I configure this kind of account to "Download full messages" or "Headers only"?
If there is only one data file with calendar and contacts folders, they will go into those folders. You can switch default email accounts - just don't change the default data file. That will create a new set of calendar and contacts folders and outlook will start using them.

My recommendation with outlook 2013 imap is to set up the profile identical to outlook 2010 and older: using a pst file as default for calendar and contacts. It's much safer as the 'this computer only' folders are not recoverable.
Is it possible to transfer all the data (contacts, calendars, notes, tasks) from one .pst\ost file to another .pst file?
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