I have heard several possible explaination. I do not think anyone has proven details as of this time. The most likely explaination is that the wad base expansion give more area for the pressure to act against than pressure loss due to the larger bore volume. I doubt the friction loss idea, but don't discount it. HMWPE has very low coefficient of friction agains steel, so I expect friction force to be low relative to gas pressure force. But, no, no proven details.