This is from a database, where I've not enabled an auto-housekeeping agent. It had grown to several gigabytes - and I'd not noticed. Well, until I had to copy a data directory around and noticed it taking a wee while longer. Now this isnt an exceptional thing at all.
The biggest mailfile I've seen in production wa 250GB in size - some eight times bigger than the 72gb maximum size supported in another less reliable mail system.
Whilst I dont advocate doing any of this, its good to know it *can*.
Update: Volker points out quite rightly that this was a log database, and so documents only get written once, etc. So its not much of an issue. Very true. However, I have dealt with 4m record notes databases in the past, where most documents change a lot. It still works, but at that stage, I'd start looking for some enterprise-level SQL instead - and no, that doesnt mean MS SQL server..