I noticed the AP Mobile app doing this also. I barely ever use the app and it continues to use more and more storage. I think it is something the developer can address in an update because I used to have a similar problem with the instagram app on my iphone.
Tunein and iHeart Radio both do this to me although nothing like 150+ MB. And it's definitely an iOS Apps issue because if I use Tunein or iHeart on my Android devices the cache is stuck at roughly 500K and the app never grows in size.