It's a balance of mass (bolt), spring rates, velocity of the bullet in the barrel (this affects time under the curve), and how much ooomph, in both volume and pressure, is needed at the piston. There are other factors, but this is a simple version so I can understand it.
The shape of the Op-rod will lend itself to bending, and even ol' John C. said it could use improvement, but the Government took it as it was.
The version we know and love so well is an adaptation of his original design for a smaller caliber, but Gen. MacArthur (SP?) was adamant about it being in .30-gov.
