I believe the difference is that the JS version specifies compression strategy 3 (Z_RLE)[0][1], whereas the Rust crate is using the default compression strategy[2]. Both otherwise use the same underlying compression library (deflate aka zlib) and the same compression level (9).

[0]: https://github.com/pretzelhammer/using-rust-in-non-rust-serv...

[1]: https://zlib.net/manual.html#Advanced:~:text=The%20strategy%...

[2]: https://github.com/rust-lang/flate2-rs/blob/1a28821dc116dac1...

Edit: Nevermind. If you look at the actual generated files, they're 594 and 577 bytes respectively. This is mostly HTTP headers.

[3]: https://github.com/pretzelhammer/rust-blog/blob/master/asset...

[4]: https://github.com/pretzelhammer/rust-blog/blob/master/asset...