Same trouble here. Since I have asinged cache to D:, you should not put any song data into my C:. It is a 128G SSD, and Spotify put 10G in my AppData\Local\Spotify\Data. That is ridiculous big, which I try to prevent from because it will hurt my SSD. Please fix this, don't put cache to my C: which I did not asinged to.
edit it with any program, even wordpad (don't use notepad) and enter this option in at the bottom of the list:
storage.size=*desired amount of megabytes*
Here's mine set for just over 1 gig:
This works! You don't even need to manually delete the files in Data, once you edit the prefs file the next time you start Spotify it will automatically delete the necesasary files to get down to the specified size limit. One thing to note, the text file is not named "perf" but rather "prefs". So C:\Users\[your username]\AppData\Roaming\Spotify\prefs.
Still, Spotify if you're listening, this is poor UX and all of this cache should be stored in the user-specified cache in the Preferences not in a non-customizable secret location!