I'm a fairly thin guy (bmi around 22-23 but most of my body fat is on my lower body, so my upper body appears more thin than what my bmi says) and had top surgery 3 years ago. Since then, I don't notice any such movement in my chest, except from when I jump on a hard surface and slightly when I run as fast as I can. Meaning, I only notice it a bit when I move around in ways that shakes that area of my body a lot. When biking, regular running, jogging and such I don't feel that kind of movement. I had a B-cup in size before my top surgery, but my surgeon insisted I had a C, but I'm sure that was incorrect. Also I don't have any defined muscle mass, it's just very slightly visible pecs and biceps, but nowhere else on my body. I'm not really working out a lot, but it happens on rare occasions.