Why my chest get bigger when I breathe out?

When you breathe out, your diaphragm and intercostal muscles relax, allowing your lungs to deflate and air to be expelled. This can cause your chest to appear to get bigger temporarily due to the expansion of the ribcage as the muscles relax. Additionally, the release of air can create a slight change in posture, making your chest appear more prominent. This phenomenon is normal and part of the breathing process.