Phone storage issues

[Firetastic]

I have an LGK10 which is about 5 years old. My storage is full. I have deleted videos which brings it down a bit. Yesterday I uninstalled my Yahoo mail app which brought it down from 15.94 to 15.25. Brilliant I thought. Then I charge phone and it reboots. Does this once in while. I look at storage again and it's back up at 15.60. In the red again. I don't understand how it could go from 15.25 to 15.60 just by a reboot. Any ideas would be gratefully appreciated.

 

[Firetastic]

So I've got a question. How do I properly back up my Whatsapp messages, some text messages and QuickMemo's that I have got? I've seen people lose their Whatsapps when going on to a new phone. Is there any software that can back up text messages and Quickmemo's? That's the only thing that is putting me off formatting it.

 

[Firetastic]

I've seen in my settings about Cached Data. When you press it it comes up and says Clear Cache Caches for all apps will be cleared. Clear? Cached Data is taking up 9.55mb. Is that quite big?