I am 230 lb and have no issue standing (other than needing a pull rope to get up, but I think that is more related to my knee). No issues with casting in flat but moving water. I regular turn my feet sideways and twist my upper body to get in a few more cast. In choppy riffle water I seat down as I don't fully trust myself to pay attention rather than fish.
That said, I have paddled whitewater for quite a few years, and been fishing out of canoes/kayaks for even longer.