If your young enough, where the cartilage between the ribs themselves is still in a state of flux, the rib box can be expanded. The mean average is from 22 to 24 years old before that cartilage begins to mature and harden. Even older men have improved the rib box (giving a fuller feeling) to a certain degree, but not to the level of a younger person. So it's a matter of the cartilage allowing a stretch between the ribs, rather that the rib bones themselves growing.
A heavy breathing exercise, like the squat primarily, followed by a set of light to moderate pullovers (either straight or bent arm) are the usual method. The attention to the stretch and deep breathing is the most concern when doing the pullover. Weight is not the issue. The squat and pullover is always done for higher reps...20 all the way up to 30 reps at times. Some people, rather than the squat, have used DL's, SLDL's, cleans, etc to induce heavy breathing. I believe that the squat would be the best way, because it allow you to fully expand the chest with full deep breaths, no restriction at all. The basic way is to take 3 deep breath's between each rep of squats. When reaching up to 12-15 reps you may be taking 5 to 7 deep breaths between reps. You should be puffing like a steam engine.
In any event, just including pullovers (bent or straight arm) in a workout can affect the ligament attachments to the chest, giving a fuller feel to that area. Pullovers also affect the lat's, tricep's and pec's (lower) strongly. Good Luck.
