The title says most of it. I appear that Outlook is not taking conversation history from the History Spooler folder (.hist) when they are bigger then 1 MB which, if you don't close the conversation, is fairly easy to reach.
We are using Lync 2013 and Outlook 2013 with on-premise Exchange 2013.
I cannot see any Error anywhere in the logs including; Lync Logs, Event Viewer, Exchange Logs.