As far as I remember, there's a lot of machinery involved to make a C call because C doesn't know anything about Go's goroutines and their resizable stacks. So

As far as I remember, there's a lot of machinery involved to make a C call because C doesn't know anything about Go's goroutines and their resizable stacks. So the runtime has to prepare the stack accordingly every time you make a call - and it's done on a dedicated CGO thread with a traditional stack.