Some foods, even though they're considered very healthy, carry loads of calories in a very small amount of food. We call these calorically dense foods and if your diet is comprised of a bunch of them, you can easily gain weight even without eating "a lot" of food.
Here are some "healthy" examples of calorically dense foods:
1. Granola- Granola, especially the varieties mixed with nuts can pack as many as 500 calories per cup!
2. Pasta- A moderate 1 and 1/2 cups of most pastas yield more than 60 grams of
carbohydrates and almost 350 calories.
3. Avocado- Avocado is awesome and a great source of monounsaturated fat, but one
single avocado is over 300 calories and 30 grams of fat.
4. Nuts and Nut Butters- Nuts are super healthy, but one of the most calorically dense
foods around. A few ounces could mean 400+ calories.
5. Fruit Juice and Smoothies- All fruit juices are loaded with sugar and so are most "smoothie" shop smoothies. Make your own with whole fruit.
6. Dried Fruit- Dried fruits remove the water content which dramatically decreases
volume...what's left is high in sugar and very calorically dense.
7. "Whole Wheat" Breads- Even the 100% whole wheat variety can pack a mean calorie
punch if you're eating a lot of grains as part of your diet.
8. Whole Grain Bagels- A large "deli" bagel is loaded with carbs and calories, many times
over 400 calories in a single bagel.
While some of the foods above are only "thought" to be healthy (fruit juice, whole grain bagels, etc.), stuff like nuts, nut butters and avocado are foods that are recommended in just about everyone's diet and they are indeed great choices.
That said, these calorically dense foods require that you monitor your intake of them closely. A few ounces of nuts, a couple tablespoons of nut butter, and and avocado is NOT a lot of food, but if you ate all of these every day, you'd be getting close to 1000 calories just right there.