I have checked the breakdown of data used. Under Settings > General > Usage, the Storage Breakdown for apps is: Photos & Camera - 387MB Whatsapp - 378MB Voice Memos - 334 MB etc. (in descending order)
In addition I have cleared most of pics and videos under Camera as part of my weekly routine. But I am still left with only 1GB (both registered on iTunes, Settings > General > Usage, and iExplorer (3rd party iPhone explorer).
What would be the reason why this is so? and how do I regain more space?
I do not download many apps. But my phone is jailbroken. Maybe it is because some Cydia apps are merely "uninstalled"? is that how it works?