GRAPHICAL REPRESENTATION
For those familiar with calculus, these rough graphs will simplify things a great deal.  They aren't to scale.  They are generalizations that will hold true for any gun.

These first graphs represent the relationship between chamber volume and muzzle velocity when barrel volume is held constant. 
As a spud travels down a barrel, the pressure behind it drops.  The bigger the chamber, the lower the pressure drop.  The less the pressure drops, the more the spud is accellerated.  When the chamber gets very large, it's almost as if pressure is constant all the way down the barrel.  The maximum theoretical velocity for the gun would occur if there were no pressure drop.  In the graph of V vs. C,  velocity approaches a theoretical maximum as chamber size increases and the pressure drop approaches zero. 
        
Note that V can never reach the theoretical maximum because there will always be a slight pressure drop no matter how large the chamber.   

If you're not familiar with calculus, the graph on the right represents how much each extra unit of chamber volume contributes to muzzle velocity. With a small chamber, each extra cc (or cubic inch, or cubic whatever) gives a big velocity boost (dV/dC is high).  With a big chamber, an extra cc gives very little boost (dV/dC is low).  Since muzzle velocity can never peaks at it's maximum, dV/dC never actually reaches zero.  Basically, this means that there isn't a certain chamber volume that will yeild maximum results.  Once dV/dC gets very close to zero, a bigger chamber will only get you slightly closer to the theoretical max. 

Maximization becomes a subjective question.  You must set a limit for dV/dC.  One person might say the gun is maximized when dV/dC falls below 1.  Someone else might choose 0.1 or 3.  With a constant barrel volume, it all comes down to  what you think is good enough.

These next graphs are another story.  They represent the relationship between BARREL volume and velocity when the CHAMBER is held constant.
As you can see, velocity has an obvious maximum accompanied by a zero in dV/dB (red dot).  At any point after the red dot, dV/dB is negative, meaning that an extra unit of barrel length will DECREASE velocity.  There is nothing subjective about it.  When you start with a fixed amount of energy, there is a single point at which the gun is maxed out.
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