Enlargement above the breast and below the collarbone may be caused by subcutaneous soft tissue mass, mass on the external ribs, lymph node metastasis of breast cancer and so on. 1. Subcutaneous soft tissue swelling: it may be triggered by factors such as lipoma, sebaceous adenoma or myofibroma, and the common symptoms of these diseases will show a lump. If it occurs above the breast and below the clavicle, it will cause localized enlargement. 2. Lump on the external rib cage: it may be caused by costochondroma, costal neuritis, which leads to lump on the external rib cage, resulting in swelling above the breast and below the collarbone. 3. Breast cancer lymph node metastasis: when patients have a history of breast cancer, they should be alert to the possibility that subclavian swelling may be related to subclavian lymph node metastasis of breast cancer. There are many reasons that can cause breast enlargement below the clavicle, it is not caused by the above factors, but also may be caused by primary lymph node disease, lymph node metastasis of other part of the tumor, chest trauma and other factors, and need to go to the hospital in time to do ultrasound, X-ray and other imaging tests to determine.