Why does Microsoft Outlook pull up 7,000 emails via IMAP and the database swells up to 20 GB?

Question

IMAP

There is a box on mail.ru belonging to the department, everyone goes there via the web interface, read / write / sort mail. Why so - do not ask, this is a "legacy solution", so everyone is used to it and everyone is comfortable.
Now everyone has a personal mailbox [email protected] (corporate Yandex), but everyone in the department needs this old mailbox. When setting up Outlook, we encountered a problem that the Outlook email database ([email protected] file ) swells up to 22 GB , which turned out to be a complete surprise for me, only the email headers should have been pulled up using the IMAP protocol.
Where such a size comes from is completely incomprehensible. More precisely, it is clear that Outlook pulls up the entire database of letters with attached files (which coincides with i-no traffic and the mail.ru report on the size of the box), it is not clear why Outlook works this way. This box needs to be set up for 20+ people ...
Well, the traditional question - how and what to do?
PS: Outlook Version 14.0.6023.1000 (32-bit)