Well since you're presumably out of puberty to be posting on this site, I'm guessing that your breast tissue has stopped developing so this new excess of boob is in fact made up of fat. From personal experience, my breasts are the first place I put weight on before my back size increases so I'm guessing that you've simply put on a bit of weight and losing a little will make them smaller (if somewhat saggier, I'm afraid).

Don't worry - you should count yourself grateful that your body distributes it that way, as not only is it more attractive but it's also healthier than putting weight on around your middle.

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