Basically, from a physiological standpoint, if you have one drink slowly, you'll probably digest most of the alcohol as energy. Drink heavily, and the calories don't "count" as much, because you'll end up excreting them.
A calorie is a measure of the energy value of a food or drink. Liquid calories are often easier and quicker to consume and will satisfy thirst, but not hunger. Liquid calories are considered “hidden” calories because the body does not feel the same fullness from a drink as it does from solid foods.
According to the American Society for Clinical Nutrition (ASCN), regular consumption of sugar calories in liquid form is said to be responsible for body weight gain. Liquid calories are often referred to as empty calories because they do not provide nutritional value.
Why? Well, for one, it's much easier to quickly consume liquid calories than it is to chew solid foods. Many of us have little trouble downing a can of soda in seconds.