I think you should increase phone storage (adding SD card) or install some app to free the storage like CCleaner or Clean Master (you might be familiar with these application for PC )
+ CCleaner: https://play.google.com/store/apps/d...ccleaner&hl=en or Download CCleaner for PC - choilieng.com
+ Clean Master: https://play.google.com/store/apps/d...r.mguard&hl=en or Download Clean Master (Boost & AppLock) for PC - choilieng.com
These apps also help your device run faster. I just installed Clean Master for my Sony Z3 and CCleaner for my HTC Desire. Both work well.