When looking at the cluster page, I can see Disk Usage = 1.6GB, for example, but I don't know what the breakdown of that is. For instance, if I look at the same DB and add up the size of the collections themselves it's barely 1.5MB. So 1.6 Gig disk = 1.5 Meg data. I suspect this is oplog etc. but it is a pretty confusing potential red herring when diagnosing performance issues etc.