http://www.myfitnesspal.com/food/calories/mcdonalds-big-mac-...
Combine it with average daily average intake:
https://en.m.wikipedia.org/wiki/List_of_countries_by_food_en...
Let's take India for example. So 2380 calories a day. The big Mac will constitute nearly 50% of calories in a meal.
Then people do have other meals too - breakfast, lunch and snacks. That means the calorie count is near to 3k calories. So a 700 calories or 30% in excess to average calories consumed.
If this happens for say only for weekends ie 8 days in a month. That is 5600 calories in excess. Without any exercise this is going to hit hard on the waist line and weight.