The average League game is about 10-30 MB, maybe a bit more.
"Our patches as you can imagine are pretty chunky at around 100-300MB depending on content, but gameplay itself is only around 10MB if I remember correctly for an average game. This can fluctuate a bit, but at a massive stretch, I'd say 30MB is the most an individual game should be using."
It's probably things you have going on in the background that's consuming data (like Steam/Origin/any client downloading updates/patching a game, Windows downloading updates in the background or a YouTube video or even someone else on your network).