Armpit fat or something more serious? (Photo)

Thank you for your pictures and excellent question.  The fat in this pre-pectoral area of the armpit is known as the Tail of Spence.  
When breast tissue comes in at puberty, it comes in like a comet having both a body and a tail.  This tail of breast tissue degenerates into breast fat early in the teenage years.  The Tail of Spence is mostly breast fat, but some actual breast tissue or gland remains for many women into their adult life.  Any breast tissue in the body can become cancerous.  The danger in the Tail of Spence is that some patients and physicians forget, in the monthly or yearly exams, to check this area for lumps.  The fact that you have a larger breast on the right side may mean that larger Tail of Spence on the same side would not be unusual.  
All the above being said, I highly recommend you see a physician to have this area checked by a professional to see if any of this tissue feels questionable.  Many plastic surgeons liposuction this area at the same time as doing a breast enhancement surgery.  Debulking this area of fat makes any discovery of unusual lumps much easier.  

this can be improved witth liposuciton and internal heating, If there is alot of loose skin It may need to be surgically removed. I would consult with us to go over what is best. see link for homonal optimizatoin and weight loss which helps enhance the results of any surgical procedure. Dr. Emer.

Dear olive101,
it is hard to tell for sure without an examination. If you are concerned, I would suggest you to get an examination. Only after a thorough examination you will get more information and recommendations. In most cases it is either fat tissue or accessory glandular tissue. 
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery