I actually believe that you can change the shape of your rib cage slightly. I know this because when I grapple and wrestle a lot my rib cage became wider when you look at it from the anterior/posterior position. It's more dramaic during wrestling tho and I think that's because there is a lot more pressure on the rib cage than in Jiu-jitsu. I think it's a way of allowing your lungs to expand more to increase your oxygen intake and i think that it's also caused by mechanical forces being applied to it I'm sure that you can do exercises that can increase the size of it but it's probably a slow process like someone mentioned above.