Sugar turns to alcohol during fermentation. Guinness has a lower ABV so that means less sugar went into the mix. Less sugar means less calories.
But not really by much. A pint of Budweiser acording to the chart above is only about 190 calories. (other sources have it as high as 220). If you want to see really high calories look into some of the very small, craft, boutique-y beers we all love so much. To get all that flavor in, they add so much in the way of sugars at the beginning of the process. That means we end up with a very high alcohol or a very high caloric, or both, end product.
Just because I’m curious: Did you ask this question because you tend to think of Guinness as a heavy, rich, creamy beer?