If you haven't explicitly limited SQL Server's memory allocation, it will take all it can getand keep it until it is forced to release it. Databases are supposed to work this way, keeping data in memory to reduce the disk IO needed to satisfy requests. You can set the server memory max as one of the Server properties through SSMS. If this is a machine that does other work than SQL Server, you might want to do that. If this machine only supports SQL Server, you might want to set it low enough to leave enough for whatever OS you're running, and forget it after that.