I don't know why the app is so storage hungry, but the easiest way I've found to keep it from eating up too much space on my mobile device is to make sure it's not left running at all times. While it's running, it's constantly collecting data from the servers, and those little packets of information will sit in your storage, and they add up.

When you're not watching a stream, make sure your app is completely closed out, and not left running in the background. If you're on iOS, it's as simple as swiping up when you get to the "open apps" screen on your phone. I'm not sure how to do it on Android, but Google probably has the answer for that. However, on Android, it's also a good idea to occasionally clear your cache every once in a while, to prevent device slow-down from a lot of this stuff bogging down your system.

There's steps on how to do that on Google, as well, depending on what your device is.

Be able to disable the continue watching