Not even excellent shot placement will immediately stop a fight, all of the time.
What is required? A combination of shot placement, enough (but not too much) penetration, a bullet carrying lots of momentum (energy), and all of that energy delivered to the target.
If you do a little reasearch in Fackler and Thompson/LaGarde, you will find hints about how much energy needs to be delivered to effect an immediate stop. (There is no absolute answer to this question.)
The diameter of the bullet doesn't matter, as long as it carries with it sufficient energy, and it stops within the target (so it delivers all of its energy).
That said, I must add that intuitively I feel that a bullet of larger diameter, or a bullet that has expanded to a large diameter, will be more effective, all other things being equal.