Is there any problem with pandoc. You mention that it takes 420 MB. Is that the only complaint you have with it? Otherwise, it seems to be meeting all your goals? You could buy a bigger hard drive.
There is a markdown program [1] which seems to be very small, which will get you from markdown to HTML. From there maybe you can find an HTML-to-PDF converter that you like. You could also try org mode for emacs which can export to PDF (via LaTex I think) but I suspect that will be very large. But maybe you already use emacs. Perhaps vi has some sort of equivalent. [1] https://daringfireball.net/projects/markdown/
