I am also experiencing extreme data usage (now up to 140+ GB per day) that appears to have started in September, increased in October, and exceeded my data cap in mid-November.
I was able to trace the problem to a process running on my Mac Pro (late 2013) called “newsd”. I have found posts about this process and its behavior that go back to 2020 and occur on several versions of macOS. The work-around was to disable iCloud syncing for the Apple News application. After I did this, I still had one instance of the newsd process running wild. I am still experimenting to see what might set the process off. The only sure work-around for me is to logout of my account on my Mac. I am also able to force-quit the newsd process using Apple’s Activity Monitor application, but the process is eventually relaunched and may go wild. Windows offers similar tools (e.g. WireShark) similar to Activity Monitor to look for processes that run wild by downloading excessive amounts of data. The Cox app’s Data Usage Activities, Usage by Category section shows the data as “Unknown” which is consistent with encrypted data being downloaded from iCloud.
Post on “newsd process on Big Sur downloading MASSIVE amounts of traffic”
< https://discussions.apple.com/thread/252209292 >
The Cox techs tried but could not help much with this kind of issue. I even got a call back from tech, Kyle, who left a detailed message about information he could determine. I appreciate their effort.