Using New Outlook on Windows for personal email, calendar, and contact management
I had the same issue. Even after a restart Outlook was using over 2.5 GB of memory and not responding. I went to Outlook Web Access, deleted a few thousand messages from folders that keep status updates, alerts and such and emptied my deleted items folder. I restarted Outlook again and now it's using about 450 MB. It looks like maybe it's trying to keep all messages in memory.