Twice I have recently updated comps from win8 to 8.1 and have seen a 8GB spike in my usage both times. 8.1 is 3.6GB and if you add 400MB for other updates you get approx 4GB.
I have also recently synced 13.8GB to my Google Drive and saw a 20GB spike in usage for that day.
Does this seem OK? Just wouldn't have thought overheads would have added so much.