Outlook.OST file grows by 6MB every time I close and reopen Outlook

I am using Outlook 2007.My Outlook.OST file grows by 6MB every time I close and reopen Outlook. This is strange. If I do this in quick fashion it doesn't happen. If I keep the outlook open for some time and after that if I close and reopen then I am seeing this issue. I am checking the outlook.OST file in C:\Users\xxxxxxx\AppData\Local\Microsoft\Outlook. The mailbox size in exchange server remails same. Only the OST file grows by 6MB.