Show size of biggest folders and files in system data
Install Solid Explorer and grant it root access. This is the most feature filled file manager that I've tried. Visit the Root (/) partition, click three dots on top right and properties. There you can see a breakdown of the content, file types and top 20 files (it does take a while to index this). 



Alternatively with a terminal emulator use the common Linux tool du and sort by size:

du -ah(x) | sort -n -r | head -n (number_to_show)

I tried to find a build of ncdu for Android which would be ideal but no luck. If you have Ubuntu/Arch/some other system with a package manager installed you could simply apt-get it.