I had my top surgery after starting T. The only thing that had any effect on my pre-op chest was the binding, T didn't do any difference there for me as far I could tell, so I don't think my results from op would have been any different if I had gotten surgery before testosterone.
However I've heard that one more reason some surgeons want for patients to be on T first is because of potential fat redistribution. I had very minimal of that happening, and only in my stomach and hip region. The fat in my tits remained the same, but binding made them a little less firm. My chest was medium size (about B- or C-cup).
I think with a very small chest, there could be a slightly higher chance of fat redistributing from there when on T, and although it might be a very slim chance, it can mean you might not even need top surgery then. Or more likely, if you'd get peri, on T you might have less tissue to remove compared to pre-T. Or with working out and being on T you could also potentially reduce the size of your chest before, or without top op.
I did not work out, early on or later on T, and didn't have any detectable muscle mass, on chest or otherwise, by the time of surgery. Hopefully I will someday in the future, but I think if I will my chest would look fine with bigger pecs, but pecs might not hide my scars very well due to their position. But that I'm not sure of, cause at this level of un-fitness, I'm not entirely sure if I even have pecs somewhere in there... I had double incision though.
