What OS is this, and how complex is backup configuration for source paths, excludes, and filters?
If setup is simple and you’re on Linux, just run the du command to get the largest source file areas. Depending on the files, compression and deduplication (especially exact dupes) may throw this off.
There’s no complete equivalent of typical OS-specific file space tools, but you can study file sizes.
With added scripting, you can probably get totals as you wish, but find command might be simpler.
Visualize backup data usage is the feature request for a nice graphical view and what to do for now.
Backend quota is close to being exceeded gets into that a bit, but also looks at next step of a purge.