Hello,
I'm having this same problem and it is pretty severe as there seems to be no limit for Spotify's saved data what so ever. I have to regularly clear the data manually as it gets so big that it uses all the available memory of my phone - I'm talking about several gigabytes here.
I usually use Spotify with Chromecast and control it with my phone, but even with Chromecast use, when the phone it self isn't streaming the music, it takes only about three weeks for the Spotify's data on the phone to grow in 4Gb, and my phone will make a warning, that it's memory is running low (I don't have a lot of memory in my phone, nor SD-card either). When I'm using bluetooth and thus streaming with the phone it self, the memory warning will pop up even faster, let's say in less than a week of use. I listen a lot of Spotify and I use best quality with premium account, but I haven't downloaded any music to be used in offline mode - that would use even more of the memory that my phone is short of.
Also, the "clear cache and saved data" -setting in Spotify's settings doesn't work, I've had to clear the data from the phone's application settings. I've tried the Spotify's setting couple times when running low on memory and it has just used rest of the memory and made the phone's software to crash. As the phone restarted, the same process was loaded and the phone crashed again and this would repeat, until I was able to uninstall Spotify in the short window of time that my phone was functional before crashing again. After this I haven't touched the data setting in Spotify's settings, but I've used the phone's application settings instead to clear the saved data of the Spotify app.
So, I guess I've made my point here and this memory usage is a problem. It has been a problem for me for more than six months.
My phone is Sony Experia Z1, Android version 5.1.1 and my current Spotify app version is 8.4.28.875 armV7.