Growing? you mean the usage per day is increasing?
Here is page on what uses credits, might give you an idea of what is eating it up.
Also, you can go to your organization main page and get a summary of what the credits are being used for (storage, analytics, etc.) that might give you an idea.
If you had offline areas checked out, disabling sync doesn't always remove the replicas. Make sure there are no orphaned replicas attached to any datasets that don't need it.
R_