Apart from having to re-calculate and optimize intake timing, the biggest benefit is that with less bottom end volume it's easier to handle more restrictive silencers. It's propably due to lessened tendency for interference- or harmonic pulses in the muffler-cylinder-crankcase -complex. L

Well that's what I found out from my tests. In that sense our engine reacts to changes in a very logical way. I guess that is a good sign.
Also, if you look at F2D engine development after their silencer rule came, there is a similar logic.
But what happens between venturi and muffler outlet is a very complex interaction of various things. L

..and I know at least one case where INCREASING case volume cured some asymmetric running issues. But in our case, DECREASING helped for same thing. So I think the correct answer starts with words "It depends.."
In my opinion, to keep things logical, you don't want an additional volume of fuel/mixture bouncing about inside crankcase.

Fellows, let me say unequivocally that I have no technical background, so my question and any further comments stem only from what passes for rational thought in my head. And I have in mind an unmuffled system.
So Eddy - is the better run due to greater CC volume or strictly to smoother flow in spite of (or without regard to) greater volume?
Ref: Lauri's last stated opinion.

Let me also say that I don't plan to design engines, I just try to learn new things to "keep an active mind". And questions way beyond me often pop up during periods of rumination.
So if that ST runs better with either more OR less cc volume there must be factors involved other than volume and flow efficiency. Or is the lesser volume crank used in combo with the greater volume/smoother flow backplate?

Your interest is appreciated and certainly of value to me, Fred - I always hope my sometimes foolish ventures into waters over my head don't just bore people to tears.

Two things, Fred.
Firstly, I do have a particular engine in mind but, with further information from folks who know about these things, it may turn out to be such foolishness on my part (wouldn't be the first time) that I don't want to mention it yet.
Secondly, it was, in fact, the stuffer backplate's successful effect (beyond rod control) that suggested the question.

Again perhaps foolishness, but I'd kinda hoped for something like "Oh, yeah - a swept volume:CC volume of N:N+.N is the best general compromise for stunt".
My alligator imagination can easily overtax my chihuahua brain. 

One question: Is the crank tunnel considered part of the CC volume?

It's been noted that with the Enya 45's, the difference between the 6001 and 6002 is mainly the tunnel size, and that sleeving the 6002 crank tunnel down to match the 6001 results in a much better stunt run. I would associate that with decreasing the CC volume resulting in better pumping action seen as increased velocity of the fuel/air charge into the CC.

Does that make sense?

In a static, non running system then most definitely the entire volume that is valved shut by the piston and inlet cut out or 'trapped' is considered the crank case volume, including any ports and the crank tunnel.
Why? Because a volume is defined by all points being at equal pressure, and its not hard to imagine a static model engine containing fluid that has definite limits.

But ........... in a dynamic running system I am not so sure since there is never equal pressure anywhere, the whole system resonates or pulses, the upper and lower volumes are linked during transfer and if you consider a full engine cycle then the volumes step outside of what is  considered discreetly  'trapped.'
If this was not true then the engine would not have a power curve - it would be much more of a flat line.

The Enya 45's crank tunnel reduction in diameter I would say has more to do with an increase in gas speed and a reduction in condensation, since the tunnel is a high speed centrifuge that will fling sheared fuel droplets against its outer walls - the bigger the walls the more the area,  the higher the surface speed and more it acts as one collecting droplets and joining them into a homogeneous mass that is more difficult to ignite.

Sure it does improve the pumping action somewhat but it is not a high performance engine that needs that a critical consideration.

Hoping to make sense, Chris.