According to point #4 the .50 pellet will need about twice the force exerted to its base in order to overcome the friction and start moving. BUT this force will be reached much sooner along the chambers pressure/time curve because double diameter results in 4 times base surface (ð x R^2) thus for a given pressure the force exerted to the pellet will be 4 time larger.
If we take this as a fact and apply Newtons Second Law: Force=mass x acceleration, we will find that the larger pellet will receive 4x more force at its base will accelerate 4 times faster in the barrel than the smaller but equally heavy one. But everybody knows this does not happen
Why? There are many theories might contribute to the real life performance. Like I said the larger pellet will develop about double the amount of drag due to friction in the barrel. Second, I am guessing that since the larger barrel has a higher internal volume, the pressure will start dropping much faster behind the pellet, as the pellet travels along the barrel.
Bottom line, the result will be similar fps @ similar grains, thus similar energy which means similar efficiency, which means similar residual fatigue on the spring.
Hope I didnt tire you unfortunately my maths are no up to the task for a more scientific analysis.