Plants and animals have an important use for all fats - to store energy. (Calories are a measure of energy, if you didn't already know.) As such, there is no such thing as a low-calorie fat. Monounsaturated, saturated, polyunsaturated, and even trans-fats are going to be high in calories per gram. It's the "per gram" part that is important here. The total calories you eat doesn't depend so much on the type of fat, as it does on the amount of fat.
Consider - if you eat a delicious organic treat that has fifteen grams of "healthy" monounsaturated fat, and your friend eats some highly processed garbage that has two grams of trans fat, another gram of unsaturated fat, and another 2 grams of monounsaturated fat to boot, who has eaten more fat? (That would be you.) When you are counting calories, the math is simple: there are roughly nine calories per gram of fat. That puts the score at You - 15 X 9 - 135 calories and Your Friend at 5 X 9 = 45 calories. To reiterate then, it isn't the type of fat, but rather how much fat that determines how many calories you're eating.
You also have to recognize that fats are not the only source of calories in a food. Proteins and carbohydrates (sugars) contribute too. So does alcohol, if any is present. A cookie made with a sparing amount of lard (saturated fats) could easily have fewer calories than a "healthy" cookie made with monounsaturated fats with sweet candy (sugar) sprinkles. More to the point, a bag of sugar has far more calories than a spoon of said lard, or any other fat. Amounts matter a great deal, when counting calories.
Now, what advantage is there then to be had from monounsaturated fats? There are several. To begin with, both polyunsaturated fats and monounsaturated fats are preferred when it comes to maintaining a lower cholesterol level. Saturated fats are linked with high cholesterol, so foods that stick to monounsaturated fats are less likely to send your cholesterol levels shooting skyward.
Your body actually needs some fat in your diet too. That definitely includes monounsaturated fats, but polyunsaturated fats are quite important too, and it's even best to have a little saturated fat in your diet too. Your body makes use of fat for all sorts of tasks, including padding your joints and muscles, and building the membranes for your cells.
If a fat is destined for use as an energy reserve, rather than a structural component of your body, then unsaturated fats are superior because they can still have a little more chemical energy added on. "Saturated" refers to how many carbon to hydrogen bonds there are in the fat. In a saturated fat, every possible hydrogen is attached. In a monounsaturated fat, there is one hydrogen "missing". In a polyunsaturated fat, many hydrogens are missing. Hydrogens are added to those sites in order to stored more energy. Doing this also consumes energy. In short, calories must be burned if the body is going to store energy in an unsaturated fat. If you start out with saturated fats, they're already full, and the body just tucks them away as extra padding, with no extra calorie burning necessary.
In the end, foods with monounsaturated fats aren't always going to be lower in calorie, but they are still usually a healthier option than foods that focus on saturated fats. You can make sure by reading the nutritional information labels before you buy. If you just want low calorie, look at the total calories box. For lower fat, look at the total fats. Check out the fat breakdown if you want to know which kinds you'll be eating. Look at the grams though, not the percents. Percents have little to do with calories, and lots to do with how much a bunch of bureaucrats decided an average-sized person should eat in their averagely-active life. On average, people just don't quite fit into that model.