I published the article below showing the pectorals can push the arm across as well as pull it toward the chest (Abduct and Adduct).
Article The Role of Compressive Force in the Movement of the Pectoralis Major
I used the same explanation for the diaphragm, but the reviewers seem skeptical that the chest expands when breathing. I know of no conventional explanation for why this happens. But it is easily varifiable, I wasn't expecting push back for that fact.
Deleted research item The research item mentioned here has been deleted
This is the current conventional answer.
Loring SH. Action of human respiratory muscles inferred from finite element analysis of rib cage. J Appl Physiol (1985). 1992;72(4):1461-1465. doi:10.1152/jappl.1992.72.4.1461
During the breathing process chest wall and diaphragm play measure role in breathing. kindly read about the bucket handle movement of ribs for your answer.
Sachin Sharma The problem I have with the description of Bucket and Handle movement of the ribs and the Pump Handle explanation, is that they are bullshit.