. . . of two factors. As Fernando mentioned, the spacing of the prong holes is critical, and unfortunately, there's not much that can be done if a particular strap doesn't have holes punched where one provides a snug fit. Tightening up a notch is not always possible. A millimeter or two might not sound like much, but it really matters.
The other factor is the texture of the strap. A pliable leather will 'give' a little, which again might not seem like much, but really has quite an effect. I've had three straps made with a particular section of cayman (over shell cordovan bottom layer) where the legs meet the torso; this 'filet' is notably more supple than the rest of the hide.
This is one of them . . .
