If you're counting KBs and you have to include zstd yourself (a reverse proxy isn't handling it for you) does that mean you're targeting an embedded device? Are you supporting TLS at that size?
39kb includes the zstd decoder which is statically compiled into it. I'm not targeting microcontrollers, but I am targeting smartphones in areas with bad internet connections, so every kilobyte saved is nice. I also just like to optimize things and make them minimal while still being powerful. I don't handle tls, GitHub pages handles all that, I'm just using it to host a static pwa.