I'm on an M1 mac, and I followed the instructions above (cargo build --release && du -sh target) and it took 6 seconds, and the target dir is 35MB. I ran it twice to make sure I wasn't pulling the remote dependencies

go.mod

    module json-test

    go 1.21.1

main.go package main

    import (
        "encoding/json"
        "fmt"
    )

    type Foo struct {
        Bar string `json:"bar"`
    }

    func main() {
        var foo Foo

        json.Unmarshal([]byte(`{"bar": "Hello, World!"}`), &foo)

        fmt.Println(foo.Bar)
    }

time go build && du -sh .

    go build  0.07s user 0.11s system 348% cpu 0.051 total
    2.4M .

I'd say 15x larger and 12x slower "bigger and longer" at least.