sudden increase in data usage since speed increase

My data usage for Sept. just about tripled from the prev 9 months.     Can't understand why as I did only slightly increase data by viewing some youtube videos on my smart tv connected via ethernet.

My computer is also connected via ethernet so outside hacking of wifi isn't the issue since I'm not connected at all via wifi.    In fact,  I have my own Motorola modem,  and the button on the back to turn wifi on and off,   is always set to off except for the rare occasion that I connect my smart phone on wifi.   I can test it so it is turned off just about always.

This seemed to coinside with the speed increase from 150 to 200 mb download but I can't be sure if that's the cause.

Has anyone else had this issue?

Already today,  Oct. 1, 2019,,   I'm at 84 GB with only a short glance of a youtube video and my computer and tv are turned off during the day till after work late at night.