Using OmniDiskSweeper (as suggested in other answers), the culprit for me was **GarageBand**. The download of Essential
Sounds said "2.1 GB", yet the size on disk was 24 GB and likely going to 50 GB (because I paused the download at 1.2 GB for lack of disk space). That explains why my disk space kept growing
even as I purged more of it.

<pre>
$ cd /private/var/folders/q8/n9hvxyg92c98xmp5bqsxyrr80000gn/C/com.apple.garageband10/com.apple.MusicApps/audiocontentdownload.apple.com
$ du -sh lp10_ms3_content_2016/
 24G	lp10_ms3_content_2016/
</pre>

I also found out that [Homebrew keeps the download of installed versions](https://github.com/Homebrew/brew/issues/3784):

> Note downloads for any installed formulae will still not be deleted. If you
> want to delete those too:
>
>     rm -rf $(brew --cache)
>
> Thanks for the answer. What's the use of keeping already installed formulae
> downloads files around?
>
> In case something goes wrong and you have to reinstall.

I had the Glasgow Haskell compiler `ghc` installed for unknown reasons. Removing it freed another 3 GB, 1.5 GB for the cached download and 1.5GB for the binary files (in `~/Library/Caches/Homebrew/downloads` and `/usr/local/Cellar/` respectively).


  [1]: https://i.sstatic.net/4aTNX.png