It's caused by air trappage (probably my own made up word). Many will also have rounded shoulders which is caused by all the coughing and often times will accentuate the barrel chestedness. My oldest daughter, Anna', had a barrel chest with slightly rounded shoulders. There really isn't much you can do, it may be slightly decreased by trying to keep your shoulders back and with breathing exercises. I've heard standing up against the wall, arms straight up taking in deep breaths and expelling and raised arm (hands clasped behind you and raising them) exercises, rolling you shoulders backwards, could also possibly help. My daughter's decreased some after her transplant, but the larger chest gave her an advantage to accommodate larger lungs than she would have if she wasn't barrel chested.
