Yes, I have bottled 1/2 bottles, but never noticed that they carbed to higher levels than the other bottles. I keg everything now, and running a 1/2 bottle experiment isn't interesting enough to get me to bottle a batch, just to test a 1/2 bottle.
Headspace volume can affect carbonation since at equilibrium the CO2 partial pressure in the headspace is determined by the carb level of the beer, the temp of the beer, and the amount of CO2 in the headspace initially. This can all be calculated for different headspace volume to beer volume ratios, and different assumptions for starting CO2 levels in the headspace.
Your statement about getting a fully carbed keg from 2.5 oz of corn sugar in 5 gal is typical of the anecdotal experiences I see reported. But, no one has shown any physical chem based math to show how this can occur.
2.5 oz (70.9 g) of corn sugar will create 36.4 g of CO2 when fully fermented. If you distribute that CO2 in 5 gal (18.9 L) of beer, you will get 1.663 g/L of CO2, which is equal to 0.84 volumes of carb. Beer after fermentation at 68˚F contains 0.84 volumes of CO2, assuming that the CO2 in the beer has come to equilibrium with the CO2 in the fermenter headspace. So, the total carb level after naturally carbonating with 2.5 oz of corn sugar would be 0.84 + 0.84 = 1.68 volumes. Even if the beer has not had enough time to equilibrate with the headspace, and has 1 volume of residual carbonation, that would still only be a final carb level of 1.84 volumes. Still way below the "observed" 2.5 volumes of carb. Where does the extra CO2 come from? This is the question that no one has been able to answer.
The only possible source for the extra CO2 is additional fermentation of the original beer, meaning that fermentation had not completed when the beer was packaged. You could try to argue that adding the priming sugar triggers additional attenuation in the keg of the original beer, but then you have to explain why you get additional attenuation in a keg but not in bottles. The only plausible explanation that I have been able to come up with is that folks aren't as careful about actually attaining FG when kegging, since they don't have to worry about bottle bombs.
I'll freely admit that I might be missing something, but if I am, someone should be able to point out just what I'm missing. If I do have things incomplete, or just plain wrong, I really want to know what the correct answer is.
Brew on