I've lived in both US/Europe and believe the kcal/100g is an important part of the nutrition label for two reasons: 1) Calorific density is much more important

I've lived in both US/Europe and believe the kcal/100g is an important part of the nutrition label for two reasons: 1) Calorific density is much more important then calories per portion. If the majority of your food intake is in the 300-500kcal/100g range then you are going to feel hungry more often, over consume and gain weight - the body's satiation feedback loop is calibrated for fresh, natural food (50-200kcal/100g) not doritos (500kcal+). Interestingly, natural foods outside the typical range are difficult to consume in large quantities (try eating 2000kcal of avocados or honey in one sitting). There's a good discussion of these issues in the Volumetrics book. 2) US "serving sizes" are completely artificial, to the point of being downright misleading. The food industry is essentially lying to consumers. Calorie density can't be spun.