The individual's genetic makeup mainly determines the quantity of fat you can store in various body parts. People who are overweight are more likely to have subcutaneous fat deposits grow in the area of their underarms. In addition, if you gain weight, you may notice that your breasts and the area directly under your arms grow more pronounced. You can lower the amount of fat in your armpits by working for general weight loss and concentrating on maintaining a nutritious diet, and establishing an exercise regimen.