That’s because of the relays. The more you have the more traffic you have.
You could advise them to use a more selective set of relays. That should help.
Or they could use it more on wifi, like I do. I don’t want to pay for all that mobile data.
Discussion
This. Also usually on WiFi.
I'm down to 6 relays, now, because I include two broadcasters, and I'm building my own broadcaster, so that I can get down to two (my broadcaster and filter.wine).