One thing about having a phone with limited memory is that the effort needed to maintain such resource.
Of course when I bought Xiaomi MI3 I've already considered this & bought a few OTG USB drives but some days, I just forget to bring this drives, so it's crucial to have some space available in case you needed it.
So here comes DiskUsage App where you can download here https://play.google.com/store/apps/details?id=com.google.android.diskusage
It has a very simple interface & a user friendly system.
After installation, just select which memory to scan.
After it finished scanning, you can instantly recognize which folder consumed the most memory & space.
You can use pinch gesture to zoom in or out.
Click a folder, then click show tab, it will use the default file explorer app to open that folder, then you can do whatever you want with those files, move it, delete it, etc.
I highly recommend this app for android users.