iPhone 4 iOS 7 seems to register more data used I have checked the breakdown of data used. Under Settings > General > Usage, the Storage Breakdown for apps is:
Photos &amp; 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?
Try iCleaner. This removes a variety of leftovers and other jailbreak files that can consume a lot of space. These files don't show in Usage as they are regarded by iOS as part of the system, but can be safely removed.

Cydia: it cleans up Cydia caches and temporary files, Cydia applications unnecessary files, partially downloaded Cydia packages and repo files.
[…]
Log files: it deletes log files and crash reports from known locations. These files are generally irrelevant to the user and can be safely deleted.
Cache files: it scans and deletes system cache files and databases. Some of them are rebuilt upon respring. This helps removing obsolete cache data.
Temporary files: it deletes temporary files from your device. These files are meant to be removed automatically, but they often happen not to. iCleaner takes care of that.