When do two casks of differing sizes yield the best preservation of beer?

The preservation of beer in two casks of differing sizes is optimized when they are emptied at different rates. This scenario allows for one cask to be consumed while the other remains relatively full, decreasing the surface area exposed to oxygen over time. Oxygen exposure can lead to oxidation, which negatively affects beer quality. By having two casks with different fill levels, the emptying process can stagger the exposure, enabling longer preservation of the beer in the fuller cask.

When it comes to the other options, mixing different beers may lead to flavor changes and possible spoilage depending on the mix, high temperatures can accelerate spoilage and alter flavor profiles, and direct sunlight exposure can cause skunking in beer, all of which would contribute to the degradation of beer quality rather than its preservation.