I'm gonna get kind of medical for a second, partly because I just finished going over that section of pathology, but also to maybe help some guys understand how and why girls need to trap balls a little differently when they chest the ball.
For one, the chest of a girl is not just made of fat. If you're a dad, you know that it serves a very important purpose early on and because of that, it's built of a lot of dense connective tissue with a lot of glands and basically a tube system. The chest isn't going to squish in like a pillow when it's hit from any direction. There's a little give, but at a certain point you're just trying to smoosh solid tissue where it won't move anymore against a hard sheet of muscle and ribs.
Secondly, there's also a reason that the chest is a 'special' zone of the body. There's a lot of nerves in and around the area that can respond quite angrily when irritated. Again, causing pain.
To avoid this, if you can't trap properly, a lot of girls will cross their arms or tuck them so that the ball hits that instead of the chest.
For most girls, the proper way to trap is to think of the upper slope of the chest as a shelf. As you let the ball drop onto the shelf, you bend your knees so your body acts like a compressed spring, absorbing the momentum of the ball and letting it drop to your feet. This avoids some of the sensitive nerve areas, decreases the force with which the ball hits you, and avoids compressing the tissue to a painful degree.
Of course, that doesn't mean there aren't other ways to successfully trap a ball with the chest. It depends on the physique of the girl and speed of the ball, etc. But this is usually the safest bet to bring a ball down safely and control it.