Dave the Brewer said:

So increased head space DOES matter, but only to a certain extent.

Lets run a mental science experiment...

Bottle of beer (any size) with 5% headspace.

Yeasties make CO2... CO2 goes into solution and some of the CO2 leaves solution to fill headspace.

Bottle of beer (any size) with 1% headspace (almost none).

Yeasties make CO2... CO2 goes into solution and 1/5 as much CO2 goes into the headspace.... leaving a little bit more CO2 in solution.

Bottle of beer (any size) with 0% headspace.

Yeasties make CO2... CO2 goes into solution and none of the CO2 leave solution to fill headspace. Because there is nothing but liquid to compress if the extra CO2 NEEDS to leave solution due to temperature then the pressure will go up VERY fast. Yielding a greatly increased chance for a bottle bomb.

In each of my mental examples... the Yeasties ate the same amount of priming sugar, and produced the same amount of CO2. The amount of CO2 in the bottle was the same, but the amount of CO2 in the beer went up with less headspace.

Now.. Lets take our 3 bottles and refridgerate them for a week.

In 5% Headspace bottle... some of the CO2 in the headspace goes back into solution.

In 1% headspace bottle... some of the CO2 goes back into solution.

In 0% headspace bottle... all the CO2 was already in solution. (No change, but less chance of a bottle bomb due to lower temps, CO2 solubility, and inactive yeasts).

I'm thinking that a little tiny bit of headspace is good... it gives you a CO2 balloon that allows the inexact science of priming sugar to generate a slightly different amount of CO2 per bottle, and gives the CO2 somewhere to sit and compress, thereby giving fewer bottle bombs...

But I can't for the life of me figure out why any headspace aids in adding carbonation.... by definition wouldn't it always lead to reduced CO2 in solution in the beer for any volume of gas instead of liquid?