Correct me if I'm wrong, but that's because the engine doesn't create enough muffler pressure to adequately fill the airspace in the larger tank, so the available muffler pressure is spread out across the larger (and ever increasing) airspace, thereby resulting in lower overall fuel delivery pressure?

Would the same be true if you rigged the tank in a "uni flow" setup? With this setup the muffler pressure is restricted by the pressure of the fuel in the tank, and as the fuel is exhausted the restriction is eased, thus allowing a more even fuel pressure delivery across the entire flight which allows you to tune for the lower (but more constant) pressure.