Since the joints are hold in position by muscles and connective tissue, and the skin is attached to the muscles. Any alterations in the muscles and connective tissue are not only going to result in changes of posture, but also are going to be visible in the skin.
Based on the change of tension in the muscles, the skin is going to crease and wrinkle if the muscles get tighter or sag if the muscle gets weaker. This changes are also visible with the breasts.
One of the muscle that anchor's the breast is attached to your shoulder, more specifically your clavicula. If you have bad posture you have rounded shoulders.
Twisting the shoulders internally will cause the clavicula and the muscle that connects it with the breast, to also rotate, which will make the breast hang lower.
Think of it like this, If you ever looked at picture of naked women I know I did some "anatomy research" when I was a teanager. You have probably noticed that some of them have their shoulders rolled back or had their arms lifted above their head.
It is not just a random cliche pose photographers and models use, it actually has a practical purposes. Rolling the shoulders back or lifting the arms effectively moves the clavicula and the muscle that attaches to the breast back or up lifting the breast, and provide some temporary lift for the picture. Having good posture would effectively work the same except it would be permanent.
Rolling the shoulders back lifts the chest up. If you have rounded shoulders your breasts will hang lower than they should.
When the shoulders are in the right position, in line with the spine, the chest will also be in the right alignment.
As always most postural problems today are caused by sitting, which makes the muscles in the back of your neck really tight. This causes your neck to curve forward, and with your neck also your shoulders are going to overcompensate and roll internally in order to stabilise your upper body.
So having rounded shoulders is really caused by a misaligned neck and can't be treated directly, but by fixing the neck, since the shoulders are only overcompensating for the neck.
Not to say that this will fix sagging skin caused by breastfeeding, large amounts of weight lose, or that it is as effective as plastic surgery, but still the results are semnificant enough that I thought I'll mention it.