It has 5 dependencies, one of which is optional, and another is serde itself: https://github.com/serde-rs/json/blob/master/Cargo.toml

    indexmap = { version = "2.2.3", optional = true }
    itoa = "1.0"
    memchr = { version = "2", default-features = false }
    ryu = "1.0"
    serde = { version = "1.0.194", default-features = false }

I don’t think you’re measuring what you think you’re measuring when you say it has 3GB of dependencies. But I can’t say for sure because you don’t provide any evidence for it, you just declare it as true.

If I were to guess, I’d say you’re doing a lot of #[derive(Serialize, Deserialize)] and it’s generating tons of code (derive does code generation, after all) and you’re measuring the total size of your target directory after all of this. But this is just a guess… other commenters have shown that a simple build produces code on the order of tens of MB…