I really think it is just the length allowing more energy to go into the ball and less into the air at the muzzle.
Alright, I did my own research. It turns out that a longer inner barrel + larger cylinder generally means a quieter gun because:
"Another thing to note is that longer barrels tend to give a quieter "muzzle blast" if you will, because of the increased expansion room within the barrel. Even properly volume matched, a gun with a shorter barrel relies on using a low-volume high pressure burst of air which expands much more suddenly within the barrel. The longer barreled guns have more space to accelerate the bb, and thus the air leaving the cylinder is at a lower initial pressure and loses its pressure more gradually since it accelerates the bb over more distance."
Also, Noobie did a test regarding barrel length vs. fps. He matched a 500mm barrel and a 313mm barrel with cylinders which provided peak efficiency for the two barrels (all other components being the same) and found out the 500mm barrel got 520 fps, while the 313mm barrel got 510 (although he did say that he could have improved his C:B ratio for his 500mm setup by a little, so maybe the difference is a little more). He then upped the spring power on both guns and found the longer barrel shooting 18 plus fps harder. So I guess the FPS difference between barrel lengths is actually quite minimal. Noobie also said that there was no difference in accuracy between the two barrels.
So in conclusion the advantage of using a longer barrel and larger cylinder is decreased noise (the "pop" of the BB being fired, not GB noise) and (very slightly) increased fps efficiency. I find it interesting that a longer barrel won't do much for accuracy or fps, although I guess a few free fps wouldn't hurt anyone!
Here is an example of what I just said. It's a chrono of the Echo 1 AKM and the Echo1 RPK. I'm pretty sure that they use the same spring and have similar internals, although the RPK probably has a larger cylinder. You can see that the RPK is shooting slightly (5-10 fps) harder than the AKM, and it is also has less of the "pop" noise while firing because of its 590mm barrel.