Why are Big Macs supposed to cost the same around the world?
Because of the Law of One Price, identical products are supposed to cost the same in different places. Since McDonald's applies standards for all the individual franchisees, the product is supposed to be identical even if it isn't perfectly identical. The law assumes there are no transportation costs. If lower costs exist in one place and there are no transportation costs you would move enough product until costs equalize. When the costs equalize the product price can equalize. This is not realistic for perishable products. The problems with this "law" are described in the limitations secton of the Big Mac Index entry in wikipedia.