just for you guys to know, it's like 17 GB Chrome, You Tube 15 GB
What happens is that the YouTube app decides you have a +1080P HD display (even thought it's phone sized) and 4G "fast" internet, so you must really want the highest quality video possible ... you need to tell it that lower quality is fine, 'cause no one can tell the difference on a 5" screen. Go to settings on YouTube and chose data saver (your network provider may also offer video saving options).
Chrome is likely left on and some page is constantly downloading things, possibly video. Just kill the task, don't leave open tabs running and usage will be lower. Make a note of the pages you go to and when you leave them open - you might find things like Reddit or Imgur chew up a lot.