Thank you! I'd not looked at the History Cleanup task, so I didn't know what it might do (yes, I'm rather new to managing an SQL Server, and don't have a mentor)
So, I took a look at a maintenance plan I had set up to backup several DBs on a server, and checked the msdb size. 108GB!! So I added in a History cleanup at the end, and will see if that shrinks it down tonight when the plan executes!
Thank you from a novice!