I am used to bottling my beer and do bottle conditioning. However, I just bought a few 5l mini kegs for the next batch which is currently fermenting. From what I have read from different sources, you need less sugar when conditioning in a mini keg than in a bottle (it varies from source to source, but it ranges from 75% to 50%, or even only 33% of the amount of sugar per liter that you would use for bottle conditioning.)
My problem, is that I would like to understand the reason behind this. I wasn't able to find any serious explanation, only things along the line "The amount of head space is different". I normally fill my bottles so that the liquid raises midway to the bottle neck (only a few cm^3 of air), and the filling instructions for the mini keg says to fill up to about 2.5cm below the rim. This means that the ratio of free space over beer volume is larger in the minikeg than in the bottles.
And now this is where I think things are getting counter-intuitive. In my understanding of the situation, a given amount of sugar will produce a given number of moles of CO2. Then the amount dissolved in the beer will depend on the pressure that builds up in the dead space above the beer. The pressure in this part, given the perfect gas approximation will be proportional to the number of moles of CO2 that escape from the beer divided by the volume of the dead space (p=nRT/V). Which means that the larger the dead volume is, the larger number of moles you need to get to the same equilibrium pressure, and therefore the same amount of CO2 dissolved in the beer. Consequently, given the larger dead space in the mini keg, I would expect to use MORE sugar per liter for the mini keg compared to bottle conditioning. This line of reasoning is in agreement with this answer, and the general observation that soft drinks tend to loose their carbonation when you only drink part of the bottle.
However, this in in contradiction with the general wisdom that you need about half as much priming sugar for mini keg than for bottles. This link even specifies, when mentioning filling of the mini keg that "Over filling the keg will result in reduced carbonation levels". For me this is completely counterintuitive, as less dead space means less moles of CO2 necessary to build up pressure in the dead space to reach equilibrium
I would be very happy if someone can give me a sound explanation as to why less priming sugar per volume of liquid is required with a mini keg compared to bottle conditioning.