Huge System Storage @iPhone 5s

Hi ...

I currently use iPhone 5s with capacity only 32Gb.

It was run out of space - only 4Gb left ..so I finally manage my photos from 9Gb to 4.45Gb.

But when checked total space did not update as I expected. as I free up 4.5gb by deleting photos, the capacity should added from 4Gb to 8.5gb

noticed the system storage capacity become bigger to 17Gb .. For me it does not make sense my iPhone system storage has 17Gb where my iPad that has more apps only have 5Gb.

Here's what I did :

search in google one of the way out is to connect my iPhone to iTunes on my Mac. it will reduce significantly. the system storage reduced from 17Gb to 7.5gb.

Happy with that.

But after I disconnect from iTunes - my Mac ,,.. the system storage back again to 17Gb

Please advise what should I do .. Any simple way rather than back up all data and reset ?

thanks

trisna