It is definitely not normal for the size of both sides of the chest to be different, and there are certain etiological differences for men and women: for men, the obvious difference in chest size may be caused by the long-term physical activity of one limb, resulting in different degrees of muscle development on both sides of the chest. Then there is a part that may be caused by the inconsistent development of one side of the breast. For women, the inconsistent size of both sides of the breast may be caused by the inconsistent development of both breast glands. Another possibility is the presence of hyperplastic lumps in the breast, if you want to know the cause, you need to do a mammogram to further clarify. The above are only breast problems, there are also some people who have visual differences caused by abnormalities in the development of the chest, such as abnormal development of the rib cage on both sides of the chest, resulting in asymmetry on both sides, which appears to be a different size of the chest, which is also possible.