It definitely can use a lot of storage on your phone. Currently using 25G on my phone. By far the largest usage of any app. I just deleted all audio downloads and it dropped to 14G. Now I can add back whatever episodes I want. 

app data. 
i didnt look too closely because i was pretty annoyed that my phone's storage was so bloated.
maybe it was my own fault.

billytheked

Fountain itself? I occasionally have to go through and delete old episodes, but I never noticed the app being too large.