I've never seen or heard of anyone seriously attempting to cut off breasts, probably because to do it well would be a lot harder, involving removing a lot more flesh in an area where the arms can't reach to as easily. Also, the prevalence of chest binders and how effective they are means that breasts can generally be more easily concealed for a "masculine" appearance with a loose shirt. You will see cutting and other forms of self-injury on the breasts sometimes. It's not a hard-and-fast rule, but my experience is that trans men tend to have an easier time with hormones, passing, and community and family acceptance, because it's easier to add secondary sex characteristics than it is to take them away. As a result, trans women may feel more desperate to affirm their status as women-- hence the self-castration attempts. But that is all speculation (granted, based in experience, but still speculation) on my part.