What causes the rib cage to shrink? About 7 years ago pulmonologist indicated my rib cage was shrinking. Originally, it was thought my lungs were small but tests indicated they are normal size they just don t have room to expand. Now I m having chest pain and it s not my heart . I m told it could be ribs pressing on muscles. So what is the cause and what can I do to stop this? I have an autistic daughter so need to live as long as I can. I cycle and exercise to keep my lungs functioning. Please help. Toni