Will bulking up affect a pectus excavatum?

Not really. But pectus can make the bulked up pectoralis muscles look assymetric. Also bulking up may make the pectus look worse if the pectoralis muscles as well as the abdominal muscles become more bulky, and the sunken sternum looks more sunken. Could also be coincidental if bulking up is done during teen years when the pectus is getting worse by itself.