My personal philosophy remains that it's more about shape than size. Basically, as soon as I looked in the mirror and thought "whoa, I'd do a double-take and ogle discreetly if those walked past," it was time to keep my shirt on. 🙂 Unfortunately for me, given my pre-HRT development and how quickly everything perked up and stuck out, that was at like week 3! Oh, well.
However, I do disagree with one thing - if it's a place where it's legal for women to go topless and some women do, then doing so still clearly *is* something a woman does. (On the flip side, in I think it was Virginia, a trans woman was refused at the DMV when she went to change her gender marker... and then arrested when she took her shirt off outside to protest, despite the M remaining on her license. So doing it as a form of self-expression is valid but can be extra risky for anyone who is trans, given how the justice system can treat us.)