Am female aged 40 not slim and not fat. Should I have my breasts enlarged?

Why? This is your decision. Ask yourself why you want them enlarged. Make a list of the reasons. Then research the certified plastic/cosmetic surgeons. Consult with at least two read current info on the types of enlargement surgeries. There is important information on the fda. Gov website regarrding both saline and silicone implants. There are expenses after surgery. Eliminate unrealistic expectations.
Up to you. This is a decision only you can make depending on how you already feel about what you have. Breast size is not usually related to your size.