Quesition about Lisp_Vector

[Qiang Guo]

Hi, I'm new to devel aspect of Emacs. And I'm having a problem reading
Emacs source code: in the definition of Lisp_Vector
 struct Lisp_Vector
  {
    EMACS_UINT size;
    struct Lisp_Vector *next;
    Lisp_Object contents[1];
  };

why contents's size is fixed ? And later when actually allocating space

 nbytes = sizeof *p + (len - 1) * sizeof p->contents[0];

I don't understand how this nbytes is calculated. why only (len-1)
contents[0] ? and how can the size of an array be changed ?

Thanks
Qiang