JLaw is amazing, and I am sick of the criticism she has received for not being 'small' enough for the part. She's a smart kid and wasn't going to starve herself for the role. I feel she makes a perfect Katniss - I couldn't have chosen the part any better myself. I had only vaguely heard of her when the first film came out, but I knew she was turning heads already and when I saw some scenes from the film, I was happy that she had the part. She plays it well.
It's not a criticism against her. It's a criticism against the person responsible for casting the part. You just can't make someone smaller. It was an integral part of the book. If you didn't read the books, you miss a lot. The part in the movie where the careers had her "treed" just didn't make sense as it was. Instead of making them to big to follow her, they made it so they were inept, the careers were not inept. If you read the book, you'd know that they were trained for this their whole lives. For the "careers" it was an honor to be in the games.