Katniss is described as thin in the book because, as you say, she lives in a starving area and is very active. Lawrence, however, decided to not lose a lot of weight for the part because she wanted to portray a larger, more healthy image. I don't oppose this, but I find it funny that despite this, fats are still complaining that she looks too thin. Furthermore, the characters in this article are all active, strong, athletic characters and that lifestyle creates a thin person. It makes no sense whatsoever to write or cast them as fatties.