Calories are useful, but satiation is may be a better metric. Because a lot of calories come from sugars (sauce, and bread), it is hard to equate two items. Protein is the most satiating, followed by fats, and then carbs.

For example:

- BigMac: 550 Cal, 30g Total Fat, 25g protein, 45g carbs

- McDouble: 400 Cal, 20g Total Fat, 22g Protein, 33g carbs

So you're saving 30% calories, while only losing 12% of protein.