How to debug skyrocketing monthly data usage?

The problem turned out to be with my cloud backup software. (Following up here in case it helps anyone else. )

CAUSE

The cause was my cloud backup software.

A “Continuous Data Protection” setting had been turned on by a software update, which uploads files in real time every time they change. My local email folder (IMAP) was in the backup set, and its search index -- a single ~1 GB file -- changes constantly, so it was being re-uploaded 10–25 times a day.

HOW I FOUND IT
Windows keeps a hidden database called SRUM (System Resource Usage Monitor) that logs network bytes sent and received per application, roughly hourly, for the past 30–60 days. The Settings -> Network & internet -> Data usage screen shows a simplified view, but the raw database has far more history. I copied it and parsed it with a free forensics tool, SrumECmd (from Eric Zimmerman’s EZ Tools), which exports it to CSV.

One app accounted for 93% of my household’s traffic, and the hourly data let me match the spikes to specific backup runs.

I turned off continuous backup, excluded the search index file (it rebuilds itself and is useless in a restore), and switched to one scheduled backup a day. Usage immediately dropped to roughly my historical average.