I'm built small everywhere but big on top. I'm about a size 4 except for the for the fact that I'm a D (that's after having them reduced down from DDD, no one told me they grow back since I was supposed to be down to a b).

Anyways I hate being smashed down.

I go with the medium tops which is slightly baggy at the bottom (mid section) although the elastic usually is tight enough that they don't move around etc while riding. Size small fits around the waist and up to my gals, however if I zip up a size small they tend to be not so comfy on longer ride in the neck area too much pulling.

I have narrow shoulders as well but the gals seem to make up for the extra room in the mediums.