Outlook keeps updating

Hi All

Hope everyone is well. I was hoping someone could assist me. Recently I noticed Outlook 2016 recreates the OST file at random for some of my users. Many of them have large mailboxes in Office 365 and we only have a 20MB internet breakout at the moment. The result is that my end users find themselves missing important emails because Outlook is continuously updating. This has been happening quite often recently. Any advice?