Spotify cache on 5109MB on internal SD, 4464MB on external SD
I ended up deleting Spotify completely. I reinstalled it, and now have downloaded 3GB of data. 

I usually download several playlists and albums to avoid downloading them repeatedly. I don't know why I ended up with two caches on internal and external SD, and I don't even know if both of them were used or if this is a bug. 

Now the internal cache is empty or very small, and external is 3GB. This is still less than either of the old caches, but that's because Spotify forgets which albums I downloaded before, and the current set is not exactly the same.