I am 120lb 5'4" small/size 2(usually).
The size chart is very true.
I fall right at the boarder between small and medium.
I tried both sizes.
Size small is form fitting. My body looks good with it on, I feel safe wearing it knowing it will not slip downward. However it is so tight that I cannot breath.
Size medium is very comfy, but it slips down at the chest area, I have to pull it up constantly.
My final solution: Keep the medium one and shorten the band (sewing required).