The whole shoulder area is as bad as a lame duck. But, as an excuse, it must be said that it's a very, *very$ complicated joint area. And Poser's joint handling is a bit outdated. Normally, the breast should be fatty tissue on the pectoralis major muscle. The part of this muscle that comes from the breastbone/ribs should go under the deltoid (shoulder cap), whilst the part that comes from the collar bone (clavicle) should cross the fibres of the first part. The breast should move with the arm stretching upwards and react to gravity (softbodies. pun intended) And then there are other muscles, many of them. And the scapula bone. The crease of the arm/breast is far too high, so I always have to smile when people talk about realism. Just think about all the movements a shoulder girdle can perform. It is only fixed by one small skeletal joint, namely where the collarbone joints the breastbone, all the rest are muscles! try to imitate this in a 3D mesh without having weightmaps etc, and you'll get problems like the one you noticed.

I have tried prudent planning long enough. From now I'll be mad. (Rumi)