I am now a Catholic, however a wrong or unclear answer could possibly change that statement and quite possibly those who are not a Catholic. That would be a sin against God, by way of example.
Getting breast reduction surgery, since you want a clear answer, is a lack. Because you lack a clear answer and this forum and members could provide confirmation of your very own feeling of rightness or sinfulness of a future act, you provide us a hint as to the nature of, for example, my very own personal lack of expertise in “getting”, “breast”, “reduction”, “surgery” et al. My post could only highlight the guidance of those who are actually qualified, tenured and willing in those areas regarding sin.
However, in “lack” I’m quite supreme, since I lack quite a lot, and I lack you ( puns ). If the surgery endangers your physical body, enjoyment, posture and ministry, it may be a heroic sacrifice that is unsuitable for you.
Sorry for the length of this response. I too am struggling with word reduction process, and that is a lack of mine. Please consult with your very own priest, bishop, archbishop before proceeding.