The good news, compiling libzstd with ZSTD_LIB_MINIFY=1 brings it down to 164k while still easily beating zlib (100k) in terms of performance and ratio.
The bad news, 164k is still pretty significant on the ncdu static binary:
582k ncdu-2.5
601k ncdu-new-nocompress
765k ncdu-new-zstd
Let's see if I can find other sources of bloat to throw away. :blobshrug: