Yup, it's certainly possible. After 2 years on anti-androgens and low dose estrogen (just enough to keep me within normal male levels) I'm up to about an A cup. It's not anything horribly impressive, but I have to wear baggy shirts to keep it from being noticeable when I go out in public. I never experienced anything like one breast developing entirely on it's own, but for quite a while my left breast was noticeably bigger, and it's only been recently that they've started to balance out.