I have Spotify and the same problem with the memory. But I also suspect that the problem may be motivated by the photos and videos. A few days ago I let the phone to my 7 years old daughter to record some videos and photos. When she returned me the phone were only 50 Mb of available memory, and I had left him with 250 Mb. And the Album App does not work well, occasionally I have the black screen.
Anyway if the problem is caused by Spotiy, I'm not willing to give up that application. I think it is Sony's responsibility to resolve this. I have Spotify in other devices and no problems.