Coke has a lot of sugar in it. Some people drink a small amount of coke, and get pleasure from it, and are not fat.

Increasing portion sizes (see for example the increasing sizes of servings in McDonalds menus) mean that people now drink huge quantities of this sugary drink.

The volume of drink, the quantity of the sugar, is the problem.

Two cookies are not a problem. Two packets of cookies are a problem.