Advice on “leanest” package manager for OS X?
The size of your package manager will be trivial (20 MB) compared to the data it needs for your packages (golang clocks in at 250 MB, node at 37 MB). I'd say stick with Homebrew and work with it if you find some libraries are causing large space usages.

My thoughts are what specifically are your HDD space concerns? You could install brew and the files it needs on an external volume, but my guess is you have other files that could be co-located elsewhere - even on an SD card if you can't accommodate these files on the boot volume.

Here are some target numbers for a clean install of OS X 10.11 with the store Xcode and homebrew installed.


5.4 GB for /System
5.1 GB for /private _( most of that is the RAM sleep image on a 16 GB Mac)
2.4 GB for /Library
1.1 GB for /Applications (and you could need an additional 5 GB for the full Xcode if you wish to carry that around - most of it isn't needed for command line and open source compilation)
0.3 GB for /Users


So between 14.5 GB of minimal filesystem usage to 20 GB if you carry all of Xcode simulators and SDK for iOS, tvOS, Mac OS that are easily thinned if you only want command line tools.

Basically, even if you have a high cost/low storage capacity 64 GB SSD drive to boot your Mac, it's likely not full of system or tools, but what you've brought to run.