From what I've read, the Mini is just a smaller version of Google Home, and from what I've read about Google Home it uses a considerable amount of data, even when sitting idle, and the data usage varies for each device type connected to it.
I've seen ranges from the low hundreds of MB per day to multiple GB per day.
I have no personal experience with the system, so I can't speak for it directly, though I don't understand why it would use so much data for simple things. One of the common themes seems to be, "...but the Amazon echo only uses X amount of data. Why does this use do much more?"
Again, this is only what I read from doing a little searching on Google. I used "Google Home data usage." Perhaps you can try that and see what you find.