Originally Posted by
Metaluna
Are you saying the larger sprockets on the cassette (e.g. 34T) cause flex, or the RD? I can't see where any RD would be affected by torque, since all the force is applied to the cassette teeth on the rear and top of the cassette where the chain is being pulled forward by the crank, not at the bottom where it enters the RD.
Prob the frame, as a big guy one of the first things I do to test a frame is to get out of the saddle and see how much flex there is.