Symmetra's ability to stay alive during that encounter is why her smaller frame is important. Not her damage, which would be completely irrelevant if she dies before she builds it up. Head to head, without strafing, D.Va murders Ms. Satya. Similarly, D.Va will not hit an up-close, strafing Winston 100%, while he will not suffer in the same manner. D.Va's movement penalty is a primary reason Winston (or Sym) is able to go toe to toe with her higher damage output.
And I can beat a Bastion who isn't having a great day as Winston, either. I am not going to suggest Winston is at all ideal to deal with that hero, though. That you're not even acknowledging how easy it is for a Symmetra to sashay in circles around D.Va tells me you haven't had to deal with that very much. A shielding Reinhardt or a D.Va up close is just a large target to fuel her photon gun levels and not much of an urgent threat.