to see how much the runtime contributes to the size of the binary use an empty main. for hello world you bring in the fmt package which is a relative heavyweight:

    $ cat > t.go
    package main
    func main() {}
    $ go build t.go 
    $ wc -c t
      623280 t
    $ nm t | wc -l
        1029
    $ cat > t.go
    package main
    import "fmt"
    func main() { fmt.Println("hello world") }
    $ go build t.go 
    $ nm t | wc -l
        2541