Storage space on iPhone 5s is decreasing

I have an iPhone 5s with the iOS 11.1.1 operating system. Upon having used it for about a fortnight, I discovered that the storage space had decreased by roughly 0.8GB despite the fact that I'd deleted all my messages and Whatsapp chats and had not added any custom files(music, photos, etc). I understand that this happened as a result of the cache getting occupied with junk files which is normal for tablets and computers in the course of being used. I then downloaded an app called Magic Cleaner on my iPhone which removes junk files from the cache. However, I was unable to remove ALL the junk files; I know this because once I was notified that there were no more junk files to be removed, the amount of storage space I recovered was definitely less than 0.8GB. In the screenshot that I've included, the 'System'(which is currently taking up 4.95GB) is what is gradually taking up storage space.

I've encountered this problem with my iPhone 5 as well(I believe this problem is relevant to all iPhone models and iOS operating systems).

I would appreciate it if anyone could please tell me why I am unable to recover all the space taken up by junk files in the cache(corresponding to the space registered by 'System' increasing) after the Magic Cleaner app performs a clean and how I could go about resolving this. Thanks.