There are alot of skinny people on raw food diets simply because they dont consume enough calories. With raw its harder to gain weight because alot of the food is quite low calorie and high in volume such as fruit and vegetables.
Calorie dense foods are nuts, seeds, avocados etc but you dont really want to go overboard on these either because then your diet becomes very fat dense.
Caloric intake depends on a variety of different factors such as weight, height, gender and activity levels. Many people have gained weight on a raw food diet but it all comes down to eating enough and the sheer volume of food can be too much for some.