Comment by _piif

The nix store in of itself should be highly compressible. Here's the output of "compsize -x /nix/store" on my BTRFS root with compress-force=zstd:6

  Processed 974464 files, 327323 regular extents (751261 refs), 598287 inline.
  Type       Perc     Disk Usage   Uncompressed Referenced  
  TOTAL       40%       11G          28G          60G       
  none       100%      2.7G         2.7G         6.2G       
  zstd        34%      8.9G          25G          54G

Nix also supports automatic deduplication using hardlinks which I have enabled in this case.