RE: Price psychology: At McDonald's in Vienna, two 0.25L small Cokes are cheaper than buying a 0.5L medium one.

My guess:

MCD experienced that if people buy a larger initial portion (Fries / burger / drink) these people are less likely to buy a second round (mc flurry / desert). Business wise it turned out that the highest profits were generated where they could convince customers to buy a second round, compared to a single large order.