From Wikipedia -

The burger methodology has limitations in its estimates of the PPP. In many countries, eating at international fast-food chain restaurants such as McDonald's is relatively expensive in comparison to eating at a local restaurant, and the demand for Big Macs is not as large in countries like India as in the United States. Social status of eating at fast food restaurants like McDonald's, local taxes, levels of competition, and import duties on selected items may not be representative of the country's economy as a whole. In addition, there is no theoretical reason why non-tradable goods and services such as property costs should be equal in different countries: this is the theoretical reason for PPPs being different from market exchange rates over time. Nevertheless, economists widely cite the Big Mac Index as a real world measurement of purchasing power parity.[8] McDonald's is also using different commercial which can result in huge differences for a product, whereas there is a smaller price difference between both countries. For example, a Hamburger sandwich costs only 1 € in France, and 1,50 € in Belgium, but in overall, McDonald's restaurant are cheaper in Belgium. In Estonia, the price difference between a Big Mac burger and the whole meal is sometimes as small as 3 EEK (0,20 USD), or 5% of the price of the burger alone.