This is the total byte count across all packets remaining after payload compression has been performed, the denominator used with the pre-compression count to compute the standard 1:x.xx compression ratio. Watching this alongside pre-compression bytes over time tells an admin whether a site's traffic, for instance increasingly encrypted or already-compressed media, is becoming less compressible, which shows up as post-compression bytes creeping closer to pre-compression bytes. For example, post-compression bytes converging with pre-compression bytes after a site adopts HTTPS-everywhere would indicate payload compression has lost most of its effectiveness on that traffic.