The calories in beer come mainly from the alcohol and the carbohydrates in the beer. By reducing either or both of these, the beer will become lighter. As well as being lighter in calories, light beers are typically lighter in taste also.
Alcohol is reduced by starting with a lower amount of sugar in the wort (lower OG.) By fermenting less sugar, less alcohol is produced. The carbohydrates in beer are so-called residual sugars. These are reduced by reducing the amount of non-fermentable sugars, such as by adjusting the recipe to avoid malts that contain unfermentable sugars (such as Cara/Crystal malts, and highly kilned malts). Also the brewer can adjust the temperature that the grains are mashed to produce a more fermentable wort with less residual sugar.
To see how alcohol and carbs affect the calories in beer, here's an excerpt from this table that lists the % abv, grams of carbohydrates in a 12.oz serving:
Name Brewer abv cal. carbohydrates
Busch Ice Anheuser Busch 5.9% 169 12.5g
Busch Light Anheuser Busch 4.1% 95 3.2g
The light beer has considerably less calories than the regular beer. In fact, around 80% of the calories in that beer come from the alcohol:
1 g of alcohol = 7 kcal.
1 g of carbohydrate = 4 kcal.
Volume of beer is 12oz. x 28.6g = 343ml. Volume of alcohol is 4.1 % x
343 = 14 ml
To convert alcohol volume to weight, divide by 1.25. Weight of alcohol
is 14 / 1.25 = 11.2g
Energy from alcohol is 11.2 x 7 = 78.4 kcal. As percent: 78.4/95 =
Energy from carbs is 3.2 x 4 = 12.8. As percent: 12.8/95 = 13%
(The total is less than 100% - the data doesn't account for protein, which is typically about 1g/12oz, which is about 4 kcal.)
By brewing the beer with an even lower %abv, the beer would be even lighter in calories, but possibly at the expense of flavor.