Is the large file system size from preallocations? If so, it seems a bit unbalanced with the actual db size. I didn’t have any spikes of temporarily storing a lot more data which could trigger a bigger preallocation.
And here’s a better view of the disk usage increase:
I have the same problem, y asked support for any advice or insight and they replied me that it is normal. Anyway my disk usage keeps ingreasing, even though i have only Kb in use for my data. Not so good mongo