1234567, there are two factors raising the pressure at the gas port. First, there is simply more gas due to the larger charge. Second, in order to avoid excessive pressures early on, this gas has to be created later in the bullet's travel. Since the total volume behind the bullet at the gas port stays the same, this means the gases in the bigger (slower) charge didn't have as much expansion. Less expansion means less energy transfer to the bullet, leaving more energy in the gases. In short, more units of gases with each unit of gas retaining more energy.
