I've got 2 Apple TV 4's, and they set to autoupdate as well.
I have quite a few apps on them as well, but no way in the world are they using 30GB a day. With 4 macs, 4 iPads, 2 iPhones, 2 ATV's, Netflix etc, and my daily usage is about 6-7GB a day (and that's including my daughter always being online watching YouTube, playing Minecraft etc).
I guess the only way to check is take your ATV's offline for a day or two and see if your usage drops, but I don't think it would be the ATV's. causing you your problem.